I have been using Thunderbird for a long time and yet I still did not know about the Compact feature.

A while back, i would use my Local Folders to save Junk, Draft, and Sent emails. This is really fast by default. But at some point I had a problem and reverted to using the remote folders which worked.

The problem was that Thunderbird would tell me that my Local Folders were full even though looking at them they were clearly empty.

The way this works is really annoyingly bad:

1. it accepts emails in a file, like the good old days, it uses one file for any number of emails

2. it adds new emails at the end of that file, which grows and grows as a result

3. it manages a corresponding index (.msf)

4. it may ask you to compact those folders once in a while, you can also do it manually by right clicking on a folder

There you go... I could have compacted the Junk folder and it would have deleted all the emails for real. i.e. purged them from the disk drive. Instead I had those very old emails sticking around for no good reasons.

The fact is that there is an auto-compact feature, but it looks like it does not work if your files are already larger than 2Gb. Most probably because at that point the size becomes negative as far as the compaction code is concerned, so the auto-compact never kicks in.

Anyway, I guess I never understood what compact meant. If you delete all the emails in a Local Folder, nothing gets deleted on the hard drive. Instead, the emails just get hidden. So that means you want to make sure you compact everything at least once so the auto-compact will kick in next. If you see a popup asking you whether you want to compact things, say OK. It will only remove emails that you already deleted. They do not want to do it all the time because it can be slow to remove an email in a single large file. But it is more than worth it!