iPhone 6/7/8 and iPad users on iOS10/iOS11 are facing this. Once user tap to delete the message pop up comes up in iPhone that iPhone is unable to delete the email saying "Unable to Move Message. The message could not be moved to the mailbox Trash". and the users is not able to delete the mails at all. The issue is even faced with iPhone 7/8 plus devices with latest iOS 11. The issue is on the built-in Mail app, and reported across almost all exchange server iCloud, Gmail, Hotmail.
The message popped up is not clear that why iPhone is unable to delete the email from email account. It is always recommended to upgrade to latest iOS version to get latest fix. However there some fixes or workaround available to fix the issue:
Fix 1: Change sync setting to no limit
iOS 10 : Go to settings, scroll down to Accounts & Passwords, tap outlook, tap mail days to sync, change to no limit. Restart your device. This will allow iPhone to take action on very old emails as well.
Fix 2: Refresh Setting and iPhone
Try turning the account off and on : Settings > Mail, Contacts, Calendars , then tap the account on the right, slide Mail to 'off', exit settings and go back into the Mail app, and then go back to Settings and slide Mail back to 'on'. Restart the device and check again if you are able to delete the email.
If you are not able to delete the email even now, close the mail app by double clicking on home button and swiping up the app window. Now Hard reset your iPhone by holding power down and home button simultaneously for ~10 sec until Apple logo appear. One iPhone is up again try deleting the email.
Fix 3: Delete and add the account
Delete the account from Settings->mail, contacts and calendars, then restart the iPad/iphone. After iPhone is turned on, reinstalled the account in mail, contacts and calendars and the mail.
Fix 4 : verify email settings
Go into the email settings for the account take the Advanced option, and have a look at the mail box behaviours for each of the Drafts, Send and Deleted mailbox. Please check if they are pointing to correct IMAP server directories like trash etc.
Fix 5: change IMAP setting
> For Gmail:
1.Press the Home button and choose Settings.
2.Scroll down and click Mail, Contacts, Calendars.
3.Tap your email account.
4.Scroll to the bottom and click Advanced.
5.Scroll to the bottom and tap the empty IMAP Path Prefix.
6. Enter INBOX in capitals, then click ‘Accounts’ at the top left and click ‘Done’ at the top right twice to close the settings.
7.Press the Home button and then re-open Mail.
8. You will now be able to delete or move your emails on your iPhone.
> hotmail, live.com or outlook.com user:
For hotmail, live.com, or outlook.com have different settings as Microsoft does not use IMAP, it uses Exchange Active Link (EAS).
set up mail server as POP3 for hotmail, live.com, or outlook.com on an iPhone