This quick tip is for Mozilla Thunderbird users who use Thunderbird with an IMAP account, but also use another mail user application (MUA) such as Outlook or webmail. If you’re in a similar situation, you might notice that when you delete messages in Thunderbird, they remain visible in your other MUA, until you exit Thunderbird. This can be especially impractical, for example, when you left Thunderbird running at the office and go to check your webmail from home. All of the spam you deleted in Thunderbird is still sitting there.

Fortunately, there’s an extremely simple solution to this problem. In Thunderbird:

Preferences -> Advanced -> Config Editor…

Search for “expunge”, and enable the “mail.imap.expunge_after_delete” option by double-clicking it. You need to restart Thunderbird after applying this setting.

Here are the before and after shots:

This setting tells Thunderbird to automatically “expunge” (IMAP terminology for “permanently delete”) any messages that have been deleted within Thunderbird. Otherwise, it just marks the message with a “delete” flag that many other mail clients ignore by default.

The “expunge_after_delete” option should be safe to use; your Trash folder will still work normally. This setting only controls the communication with the email server.