When I delete an email message from my inbox, it disappears for a few seconds and then immediately reappears. Here is the error message displayed, “The current operation on 'Inbox' did not succeed. The mail server for account XXX responded: Command Error.12." This also occurs when trying to delete (or even move) messages from any folder except the Trash folder (i.e. it lets me remove messages from the Trash folder). I am using Thunderbird for several email accounts, and this only occurs for this one email account, which is an imap account. I first fixed the problem by going into the Config editor and changing the value for this field (mail.server.default.send_client_info) from true to false (see https://support.mozilla.org/en-US/questions/1064915 ). This worked for a couple of hours, then the problem re-occurred. I went ahead and changed the value back to true. Next, I tried deleting the Trash and Trash.msf files from my Profile folder, just in case they were corrupted (see https://support.mozilla.org/en-US/kb/cannot-delete-messages ). That initially did not work. Then I tried also deleting the INBOX and INBOX.msf files, along with the Trash and Trash.msf files. It did not work until I restarted my computer. This fix worked for several days, but then reappeared. This first only occurred on my work computer (Windows 7 Professional) while using Thunderbird, then several days later it happened on my personal laptop (Mac OS 10.11.6) while using Thunderbird, and now (weeks later) it is happening on both. This does not happen when I access this email account with my phone (Android 7.1.1) nor online, neither of which use Thunderbird. Now neither fix works. I have even tried deleting all files and folders that do not contain actual email messages from my Profile folder (i.e. I deleted all files with extension .msf and all folders with extension.sbd). This did not work either. In addition, I used my phone (where I can still do things), and I emptied all of these folders: Trash, Deleted Items, Sent, Sent Items, Junk, Inbox. Next, I compacted everything. Then I deleted everything but actual email messages from my Profile folder. Lastly, I restarted my computer. This did not work.