diff --git a/src/components/InternalAddress.vue b/src/components/InternalAddress.vue
index 77bd6cf558..1653f7742e 100644
--- a/src/components/InternalAddress.vue
+++ b/src/components/InternalAddress.vue
@@ -62,8 +62,17 @@
 			v-model:open="openDialog"
 			:buttons="buttons"
 			:name="t('mail', 'Add internal address')"
+			:isForm="true"
+			:noClose="saving"
 			@close="openDialog = false">
-			<NcTextField v-model="newAddress" class="input" :label="t('mail', 'Add internal email or domain')" />
+			<NcTextField
+				ref="addressField"
+				v-model="newAddress"
+				class="input"
+				:label="t('mail', 'Add internal email or domain')"
+				:readonly="saving"
+				:error="errorMessage !== ''"
+				:helperText="errorMessage" />
 		</NcDialog>
 	</div>
 </template>
@@ -87,6 +96,7 @@ import IconAdd from 'vue-material-design-icons/Plus.vue'
 import IconDelete from 'vue-material-design-icons/TrashCanOutline.vue'
 import logger from '../logger.js'
 import useMainStore from '../store/mainStore.js'
+import { parseInternalAddress } from '../util/internalAddress.js'
 
 const sortByAddress = sortBy(prop('address'))
 
@@ -108,24 +118,36 @@ export default {
 		return {
 			openDialog: false,
 			newAddress: '',
-			buttons: [
+			saving: false,
+			errorMessage: '',
+		}
+	},
+
+	computed: {
+		...mapStores(useMainStore),
+		buttons() {
+			return [
 				{
-					label: 'Cancel',
+					label: t('mail', 'Cancel'),
 					icon: IconCancel,
+					disabled: this.saving,
 					callback: () => { this.openDialog = false },
 				},
 				{
-					label: 'Ok',
+					label: t('mail', 'Ok'),
+					type: 'submit',
 					variant: 'primary',
 					icon: IconCheck,
-					callback: () => { this.addInternalAddress() },
+					disabled: this.newEntry.address === '',
+					callback: () => this.addInternalAddress(),
 				},
-			],
-		}
-	},
+			]
+		},
+
+		newEntry() {
+			return parseInternalAddress(this.newAddress)
+		},
 
-	computed: {
-		...mapStores(useMainStore),
 		list() {
 			return this.mainStore.getInternalAddresses
 		},
@@ -139,6 +161,19 @@ export default {
 		},
 	},
 
+	watch: {
+		newAddress() {
+			this.errorMessage = ''
+		},
+
+		openDialog(open) {
+			if (!open) {
+				this.newAddress = ''
+				this.errorMessage = ''
+			}
+		},
+	},
+
 	methods: {
 		async removeInternalAddress(sender) {
 			// Remove the item immediately
@@ -155,38 +190,35 @@ export default {
 		},
 
 		async addInternalAddress() {
-			const type = this.checkType()
+			const { address, type } = this.newEntry
+			this.errorMessage = ''
+			this.saving = true
 			try {
 				await this.mainStore.addInternalAddress({
-					address: this.newAddress,
+					address,
 					type,
-				}).then(async () => {
-					this.newAddress = ''
-					this.openDialog = false
 				})
+				this.openDialog = false
 			} catch (error) {
-				logger.error(`Could not add internal address ${this.newAddress}`, {
+				if (error.response?.status === 422) {
+					this.errorMessage = t('mail', 'Enter a valid email address or domain')
+					await this.$nextTick()
+					this.$refs.addressField?.focus()
+					return false
+				}
+
+				logger.error(`Could not add internal address ${address}`, {
 					error,
 				})
 				showError(t('mail', 'Could not add internal address {address}', {
-					address: this.newAddress,
+					address,
 				}))
+				return false
+			} finally {
+				this.saving = false
 			}
 		},
 
-		checkType() {
-			const parts = this.newAddress.split('@')
-			if (parts.length !== 2) {
-				return 'domain'
-			}
-			// remove '@'' from domain if added by mistake
-			if (parts[0].length === 0) {
-				this.newAddress = parts[1]
-				return 'domain'
-			}
-			return 'individual'
-		},
-
 		senderType(type) {
 			switch (type) {
 				case 'individual':
diff --git a/src/store/mainStore/actions.js b/src/store/mainStore/actions.js
index e088ee309e..18c9ac0fe0 100644
--- a/src/store/mainStore/actions.js
+++ b/src/store/mainStore/actions.js
@@ -1287,6 +1287,11 @@ export default function mainStoreActions() {
 		}) {
 			return handleHttpAuthErrors(async () => {
 				const internalAddress = await addInternalAddress(address, type)
+				if (internalAddress === null) {
+					logger.debug('internal address is already internal')
+					return
+				}
+
 				this.addInternalAddressMutation(internalAddress)
 				logger.debug('internal address added')
 			})
diff --git a/src/tests/unit/components/InternalAddress.vue.spec.js b/src/tests/unit/components/InternalAddress.vue.spec.js
new file mode 100644
index 0000000000..7d7a63719c
--- /dev/null
+++ b/src/tests/unit/components/InternalAddress.vue.spec.js
@@ -0,0 +1,254 @@
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+import { showError } from '@nextcloud/dialogs'
+import { createTestingPinia } from '@pinia/testing'
+import { enableAutoUnmount, flushPromises, shallowMount } from '@vue/test-utils'
+import { setActivePinia } from 'pinia'
+import { nextTick } from 'vue'
+import NcButton from '@nextcloud/vue/components/NcButton'
+import NcDialog from '@nextcloud/vue/components/NcDialog'
+import NcTextField from '@nextcloud/vue/components/NcTextField'
+import InternalAddress from '../../../components/InternalAddress.vue'
+import Nextcloud from '../../../mixins/Nextcloud.js'
+import useMainStore from '../../../store/mainStore.js'
+
+vi.mock('@nextcloud/dialogs', async (importOriginal) => ({
+	...await importOriginal(),
+	showError: vi.fn(),
+}))
+
+describe('InternalAddress', () => {
+	enableAutoUnmount(afterEach)
+
+	let store
+
+	beforeEach(() => {
+		setActivePinia(createTestingPinia())
+		store = useMainStore()
+	})
+
+	afterEach(() => {
+		vi.clearAllMocks()
+	})
+
+	async function mountWithAddress(address) {
+		const wrapper = shallowMount(InternalAddress, {
+			attachTo: document.body,
+			global: {
+				mixins: [Nextcloud],
+				stubs: {
+					NcInputField: false,
+					NcTextField: false,
+				},
+			},
+		})
+		wrapper.findComponent(NcButton).vm.$emit('click')
+		await wrapper.findComponent(NcTextField).setValue(address)
+		return wrapper
+	}
+
+	function findOkButton(wrapper) {
+		return wrapper.findComponent(NcDialog).props('buttons').find((button) => button.variant === 'primary')
+	}
+
+	function findCancelButton(wrapper) {
+		return wrapper.findComponent(NcDialog).props('buttons').find((button) => button.variant !== 'primary')
+	}
+
+	function enteredAddress(wrapper) {
+		return wrapper.findComponent(NcTextField).props('modelValue')
+	}
+
+	it('submits the dialog form with Ok, so Enter adds the address', async () => {
+		const wrapper = await mountWithAddress('example.test')
+
+		expect(wrapper.findComponent(NcDialog).props('isForm')).toBe(true)
+		expect(findOkButton(wrapper).type).toBe('submit')
+		expect(findCancelButton(wrapper).type).not.toBe('submit')
+	})
+
+	it('translates the button labels', async () => {
+		await mountWithAddress('example.test')
+
+		expect(t).toHaveBeenCalledWith('mail', 'Cancel')
+		expect(t).toHaveBeenCalledWith('mail', 'Ok')
+	})
+
+	it.each([
+		[true, ''],
+		[true, '   '],
+		[true, '@'],
+		[true, ' @ '],
+		[false, 'example.test'],
+	])('sets Ok disabled to %s for "%s"', async (disabled, address) => {
+		const wrapper = await mountWithAddress(address)
+
+		expect(findOkButton(wrapper).disabled).toBe(disabled)
+	})
+
+	it.each([
+		[' example.test ', 'example.test', 'domain'],
+		['@example.test', 'example.test', 'domain'],
+		[' @ example.test', 'example.test', 'domain'],
+		[' alice@example.test ', 'alice@example.test', 'individual'],
+	])('adds "%s" as "%s" of type %s', async (input, address, type) => {
+		store.addInternalAddress.mockResolvedValue()
+		const wrapper = await mountWithAddress(input)
+
+		await findOkButton(wrapper).callback()
+
+		expect(store.addInternalAddress).toHaveBeenCalledWith({ address, type })
+	})
+
+	it('keeps the Ok button busy until the internal address is added', async () => {
+		let finishAdding
+		store.addInternalAddress.mockReturnValue(new Promise((resolve) => {
+			finishAdding = resolve
+		}))
+		const wrapper = await mountWithAddress('example.test')
+
+		const result = findOkButton(wrapper).callback()
+		await flushPromises()
+
+		expect(await Promise.race([result, 'still saving'])).toBe('still saving')
+		finishAdding()
+		expect(await result).not.toBe(false)
+		await nextTick()
+		expect(store.addInternalAddress).toHaveBeenCalledWith({ address: 'example.test', type: 'domain' })
+		expect(enteredAddress(wrapper)).toBe('')
+	})
+
+	it('blocks cancelling and closing while the internal address is saved', async () => {
+		let finishAdding
+		store.addInternalAddress.mockReturnValue(new Promise((resolve) => {
+			finishAdding = resolve
+		}))
+		const wrapper = await mountWithAddress('example.test')
+
+		const result = findOkButton(wrapper).callback()
+		await nextTick()
+
+		expect(findCancelButton(wrapper).disabled).toBe(true)
+		expect(wrapper.findComponent(NcDialog).props('noClose')).toBe(true)
+		expect(wrapper.find('input').attributes('readonly')).toBeDefined()
+		finishAdding()
+		await result
+		await nextTick()
+		expect(findCancelButton(wrapper).disabled).toBe(false)
+		expect(wrapper.findComponent(NcDialog).props('noClose')).toBe(false)
+		expect(wrapper.find('input').attributes('readonly')).toBeUndefined()
+	})
+
+	it('keeps Ok enabled while saving, so it keeps the focus', async () => {
+		store.addInternalAddress.mockReturnValue(new Promise(() => {}))
+		const wrapper = await mountWithAddress('example.test')
+
+		findOkButton(wrapper).callback()
+		await nextTick()
+
+		expect(findOkButton(wrapper).disabled).toBe(false)
+	})
+
+	it('keeps the dialog open if the internal address could not be added', async () => {
+		store.addInternalAddress.mockRejectedValue(new Error('Request failed'))
+		const wrapper = await mountWithAddress('example.test')
+
+		const result = await findOkButton(wrapper).callback()
+		await nextTick()
+
+		expect(result).toBe(false)
+		expect(showError).toHaveBeenCalledOnce()
+		expect(enteredAddress(wrapper)).toBe('example.test')
+		expect(findCancelButton(wrapper).disabled).toBe(false)
+		expect(findOkButton(wrapper).disabled).toBe(false)
+		expect(wrapper.findComponent(NcDialog).props('noClose')).toBe(false)
+		expect(wrapper.findComponent(NcTextField).props('error')).toBe(false)
+	})
+
+	describe('when the server rejects the address', () => {
+		const validationError = () => Object.assign(new Error('Request failed with status code 422'), {
+			response: { status: 422 },
+		})
+
+		async function addRejectedAddress() {
+			store.addInternalAddress.mockRejectedValue(validationError())
+			const wrapper = await mountWithAddress('not valid')
+			const result = await findOkButton(wrapper).callback()
+			await nextTick()
+			return { wrapper, result }
+		}
+
+		it('shows the error at the text field instead of a toast', async () => {
+			const { wrapper, result } = await addRejectedAddress()
+
+			expect(result).toBe(false)
+			expect(showError).not.toHaveBeenCalled()
+			expect(t).toHaveBeenCalledWith('mail', 'Enter a valid email address or domain')
+			expect(wrapper.findComponent(NcTextField).props('error')).toBe(true)
+			expect(wrapper.findComponent(NcTextField).props('helperText')).not.toBe('')
+			expect(enteredAddress(wrapper)).toBe('not valid')
+			expect(document.activeElement).toBe(wrapper.find('input').element)
+		})
+
+		it('moves the focus to the field once the error is rendered', async () => {
+			store.addInternalAddress.mockRejectedValue(validationError())
+			const wrapper = await mountWithAddress('not valid')
+			const input = wrapper.find('input').element
+			let describedByOnFocus = null
+			input.addEventListener('focus', () => {
+				describedByOnFocus = input.getAttribute('aria-describedby')
+			})
+
+			await findOkButton(wrapper).callback()
+
+			expect(document.activeElement).toBe(input)
+			expect(describedByOnFocus).toContain('helper-text')
+		})
+
+		it('hides the previous error while the address is sent again', async () => {
+			const { wrapper } = await addRejectedAddress()
+			store.addInternalAddress.mockReturnValue(new Promise(() => {}))
+
+			findOkButton(wrapper).callback()
+			await nextTick()
+
+			expect(wrapper.findComponent(NcTextField).props('error')).toBe(false)
+		})
+
+		it('ignores a rejection that arrives after the dialog is gone', async () => {
+			let rejectAdding
+			store.addInternalAddress.mockReturnValue(new Promise((resolve, reject) => {
+				rejectAdding = reject
+			}))
+			const wrapper = await mountWithAddress('not valid')
+			const result = findOkButton(wrapper).callback()
+
+			wrapper.unmount()
+			rejectAdding(validationError())
+
+			expect(await result).toBe(false)
+		})
+
+		it('clears the error when the address is edited', async () => {
+			const { wrapper } = await addRejectedAddress()
+
+			await wrapper.findComponent(NcTextField).setValue('example.test')
+
+			expect(wrapper.findComponent(NcTextField).props('error')).toBe(false)
+			expect(wrapper.findComponent(NcTextField).props('helperText')).toBe('')
+		})
+
+		it('resets the address and the error when the dialog is closed', async () => {
+			const { wrapper } = await addRejectedAddress()
+
+			findCancelButton(wrapper).callback()
+			await nextTick()
+
+			expect(enteredAddress(wrapper)).toBe('')
+			expect(wrapper.findComponent(NcTextField).props('error')).toBe(false)
+		})
+	})
+})
diff --git a/src/tests/unit/store/actions.spec.js b/src/tests/unit/store/actions.spec.js
index ce7f10cc76..2e9eb319b3 100644
--- a/src/tests/unit/store/actions.spec.js
+++ b/src/tests/unit/store/actions.spec.js
@@ -6,6 +6,7 @@
 import { createPinia, setActivePinia } from 'pinia'
 import { curry, range, reverse } from 'ramda'
 import * as AccountService from '../../../service/AccountService.js'
+import * as InternalAddressService from '../../../service/InternalAddressService.js'
 import * as MailboxService from '../../../service/MailboxService.js'
 import * as MessageService from '../../../service/MessageService.js'
 import * as NotificationService from '../../../service/NotificationService.js'
@@ -14,6 +15,7 @@ import useMainStore from '../../../store/mainStore.js'
 import { normalizedEnvelopeListId } from '../../../util/normalization.js'
 
 vi.mock('../../../service/AccountService.js')
+vi.mock('../../../service/InternalAddressService.js')
 vi.mock('../../../service/MailboxService.js')
 vi.mock('../../../service/MessageService.js')
 vi.mock('../../../service/NotificationService.js')
@@ -796,6 +798,28 @@ describe('Vuex store actions', () => {
 		})
 	})
 
+	describe('addInternalAddress', () => {
+		it('adds the created internal address', async () => {
+			const internalAddress = { id: 3, address: 'example.test', type: 'domain' }
+			InternalAddressService.addInternalAddress.mockResolvedValue(internalAddress)
+
+			await store.addInternalAddress({ address: 'example.test', type: 'domain' })
+
+			expect(InternalAddressService.addInternalAddress).toHaveBeenCalledWith('example.test', 'domain')
+			expect(store.getInternalAddresses).toEqual([internalAddress])
+		})
+
+		it('keeps the internal addresses unchanged if the address already exists', async () => {
+			const internalAddress = { id: 3, address: 'example.test', type: 'domain' }
+			store.addInternalAddressMutation(internalAddress)
+			InternalAddressService.addInternalAddress.mockResolvedValue(null)
+
+			await store.addInternalAddress({ address: 'example.test', type: 'domain' })
+
+			expect(store.getInternalAddresses).toEqual([internalAddress])
+		})
+	})
+
 	describe('startComposerSession reply-to resolution', () => {
 		const account = {
 			id: 1,
diff --git a/src/tests/unit/util/internalAddress.spec.js b/src/tests/unit/util/internalAddress.spec.js
new file mode 100644
index 0000000000..ff799b676a
--- /dev/null
+++ b/src/tests/unit/util/internalAddress.spec.js
@@ -0,0 +1,22 @@
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+import { parseInternalAddress } from '../../../util/internalAddress.js'
+
+describe('parseInternalAddress', () => {
+	it.each([
+		['example.test', 'example.test', 'domain'],
+		['  example.test  ', 'example.test', 'domain'],
+		['@example.test', 'example.test', 'domain'],
+		[' @ example.test ', 'example.test', 'domain'],
+		['alice@example.test', 'alice@example.test', 'individual'],
+		[' alice@example.test ', 'alice@example.test', 'individual'],
+		['', '', 'domain'],
+		['   ', '', 'domain'],
+		['@', '', 'domain'],
+	])('parses "%s" as "%s" of type %s', (input, address, type) => {
+		expect(parseInternalAddress(input)).toEqual({ address, type })
+	})
+})
diff --git a/src/util/internalAddress.js b/src/util/internalAddress.js
new file mode 100644
index 0000000000..dcf604ca3b
--- /dev/null
+++ b/src/util/internalAddress.js
@@ -0,0 +1,23 @@
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+/**
+ * Split the text entered for a new internal address into the address and its type.
+ * Spaces around the text and a leading "@" before a domain are removed.
+ *
+ * @param {string} input The entered text
+ * @return {{ address: string, type: 'domain'|'individual' }}
+ */
+export function parseInternalAddress(input) {
+	const address = input.trim()
+	const parts = address.split('@')
+	if (parts.length !== 2) {
+		return { address, type: 'domain' }
+	}
+	if (parts[0].length === 0) {
+		return { address: parts[1].trim(), type: 'domain' }
+	}
+	return { address, type: 'individual' }
+}