In the case of an imap mail server compact also executes an expunge command on the mail server. Like Thunderbird IMAP saves a lot of the housekeeping for later and expunge allows he server to really clean up deleted mail rather than, (like Thunderbird), simply hide it.

You can also expunge on exit in account settings. But it is an integral part of the compact process on IMAP accounts.

When I select one folder, and command compacting, it compacts ALL the folders at least in that one account. Does "compact folders" compact ALL the folders in all accounts on my Thunderbird? That is, when I hit "compact folders" is EVERYTHING on my TB system compacted? Or do I have to "compact folders" one account at a time?

Yes, I believe "compact folders" mainly gets rid of drafts, and other superseded messages. I believe "expunge" mainly just gets rid of messages that are marked for deletion. So you're saying that "compact folders" does it all.

If you use the compact folders command in the file menu on the main menu bar then all folders in all accounts will be compacted.

If you right click and select compact folder in a pop or local folder account then that folder will be compacted.

If you right click an imap folder and select compact then the folder will be compacted and expunged as appropriate. It is not "per account"

Note this part well compact like expunge removed already deleted (but hidden from view) mail from the mail store. It has not other houskeeping function. It certainly does not clean up drafts etc. What it does do is cause a new index to be created, so if something in the past has cause he index to become corrupt in relation to the actual mail store and show mail that really no longer exists then the new index will make it disappear. But it is not the compact process that causes this, it is a byproduct of building the new index post the compact. Choosing to repair folders does exactly the same thing. Rebuilds the index used to display mail

Ah, that makes sense. Thank you! I wasn't aware that "compact" was a right click option for an individual folder.

What I meant by cleaning up drafts was deleting drafts that have been superseded by a final copy.

Up until recently I rarely did [was asked to] compact. Considering it to be a reasonable maintenence action I looked at the settings. In the ADVANCED tab under the subsection 'Network & Diskspace' I found the option 'compact all folders when it will save over [choice] MB in total. From that moment on TB asked to compact at start up and frequently when I opened another inbox, usually stating it would save a couple of hundred KB so [thats KB], no matter how far I moved the MB choice up [max 2048 MB] it persisted. I have now unchecked that option and [after a restart] I am no longer confronted with what I consider to be a too frequent action. I shall keep up with the occaisional manual [right click] option. I suggest that the routine which decides to compact does it whenever one deletes an email, so creating something that needs cleaning up, no matter how small and independent of the ammount of storage space can be saved.