Deleted e-mails in an Office 365 Outlook account keep reappearing in the Thunderbird e-mail client

Whatever OS and e-mail client you use, if you search the Web you’ll find plenty of posts about deleted e-mails that reappear after you empty the ‘Trash’/’Deleted Items’/’Recycle bin’ folder. This problem seems to occur mostly with e-mail accounts that use the IMAP (Internet Message Access Protocol) or the Microsoft EWS (Exchange Web Services) protocols.

The obvious thing to check first in the e-mail client is if it has been configured to actually delete the e-mails on the server when you empty the ‘Deleted Items’ folder. Furthermore, internal files called ‘folder index files’ (.msf) in Thunderbird can sometimes become damaged, and these damaged files can also result in deleted e-mails reappearing. There is a ‘Repair Folder’ option in Thunderbird that sometimes fixes this problem. If that does not work, deleting the relevant folder’s .msf file and allowing Thunderbird to rebuild it sometimes fixes the problem. Anyway, I tried all the suggested approaches and more, as well as completely removing the account in Thunderbird (including ticking ‘Remove message data’ and manually deleting any remaining files for that account that remained in the Thunderbird directory). But nothing worked. However, I eventually cracked the problem as explained below.

Let’s just recap my situation:

1. Of the many e-mails in a corporate Office 365 account that I had deleted over the last few months and emptied from the account’s ‘Deleted Items’ folder, five of them would keep reappearing in that account’s ‘Deleted Items’ folder in Thunderbird. This happened if I deleted the e-mails individually from the ‘Deleted Items’ folder and if I right-clicked on the folder and selected ‘Empty Deleted’.
2. Whenever I logged in to the Office 365 Outlook account via a Web browser, the ‘Deleted Items’ folder was empty and it showed ‘Recover items deleted from this folder (0 items)’, i.e. no deleted messages existed.
3. The Samsung e-mail client on my Samsung Galaxy Note 8 mobile phone showed the ‘Recycle bin’ for the same account was empty.

All the settings for the account in Thunderbird were correct. Completely removing the account from Thunderbird then adding the account again did not solve the problem, meaning that the problem could not be due to Thunderbird or the ExQuilla add-on Thunderbird uses to enable it to access the Office 365 Outlook account using Microsoft EWS. Even though my Samsung mobile phone’s e-mail client showed the Office 365 Outlook account’s ‘Recycle bin’ was empty, I selected ‘Email settings’ in the Samsung e-mail client, selected the account, scrolled down to ‘Empty Recycle bin’ and tapped it. The following message was displayed:

Empty Recycle bin?

This will permanently delete the items in the Recycle bin.

Cancel            Delete

I tapped ‘Delete’ and the Samsung e-mail client displayed ‘Success’. The five rogue e-mails then disappeared from the ‘Deleted Items’ folder in Thunderbird. The next time I logged in to the Office 365 Outlook account via a Web browser, the ‘Deleted Items’ folder was still empty but it displayed ‘Recover items deleted from this folder (5 items)’. I used the usual Office 365 Outlook procedure to recover the five e-mails and delete them permanently, resulting in ‘Recover items deleted from this folder (0 items)’ being displayed again in Office 365 Outlook in the browser window.

So, there you have it, the problem had nothing to do with Thunderbird or ExQuilla. If you access an Office 365 Outlook e-mail account via an e-mail client on a desktop or laptop and also via an e-mail client on a mobile phone, and you find that e-mails you deleted and emptied from the ‘Deleted Items’/’Trash’/’Recycle bin’ folder in the e-mail client on the desktop/laptop keep reappearing in that folder, try deleting them on all your devices, including your phone, even if the e-mails are not shown in the Office 365 Outlook account in a Web browser nor in the Office 365 Outlook account in a phone’s e-mail client.