Like many other people, I use Linux as a backup server. The other day, I noticed that daily incremental backup of one of the Windows machines was well over 1 GB even on the day the user was mostly idle. The only thing the user was doing was … e-mailing. Aha! (heard the bell?) It must be that inflated Inbox.
Mozilla-based mail clients like Thunderbird and Seamonkey mail do not physically remove messages that user deletes. Instead they are only tagged “deleted”. This is true even after the Trash folder is emptied. The [supposedly] deleted mails get [really] deleted when Inbox (or any folder for that matter) is compacted.
I went to the blasted machine and did just that and the Inbox went from > 1 GB to a fraction of its original size.
Of course, this is not just Windows. Huge mail folders can potentially cause trouble and also degrade the performance of the client. The best strategy to prevent this is to set up an automatic clean up. In Thunderbird, go to Edit -> Preferences -> Advanced -> Network & Disk Space and then enable the “Compact folders when it will save over…KB” option.