Without context that is meaningless. Is there a more complete conversation you are extracting this from? Why yes there is. https://support.mozilla.org/en-US/questions/1424033 I suggest you look at. https://support.mozilla.org/en-US/kb/compacting-folders But be aware that when I wrote that 10 years ago it referred primarily to POP accounts. The way IMAP works, expunge kicks in for many folders server side, but compact will still be required to clear out the local MBOX store. When you empty a pop account trash folder using the menu entry, Thunderbird just deleted the relevant mbox file. so compact is not needed. If you select and press delete on all the mail in the trash folder, you still need to compact. Confusing hey. You may also find that expunge is not working for either the junk or the trash folders because they are not included in the fetch when getting mail. The fetch only occurs when the folder is selected by default. I find that rather silly, but it is a default from 25 years ago when bandwidth was particularly precious. it is also relatively easy to change by going to the folder properties. Fundamentally the whole mbox process is old, creaking and over due for replacement. I have no idea when the development team will bite the bullet and move to the [https://support.mozilla.org/en-US/kb/maildir-thunderbird maildir ]like storage that has been in Thunderbird and preferenced off pending some bug fixes for more than a decade. It is in the feedback forum as an [https://connect.mozilla.org/t5/ideas/really-finish-quot-maildir-quot-message-storage/idi-p/29251 Idea]. The initial code was written by Mozilla Messaging before Mozilla closed them down in 2011 (I think that was when) But I can assure you, unless it leads to data loss for the user, no one will be putting much effort into mbox. It's days are numbered, except perhaps as an archive format in the same sort of way as Googles takeout uses it.