I have been using Dropbox to sync my mail across multiple computers (I’m old school using POP protocol) which has been working great.
Recently I received an email from Dropbox saying I was running out of space on my free 2GB account. I found that my Mozilla Thunderbird profile which contained mainly my emails were taking up about 1.1 GB of hard disk / Dropbox space.
According to the Mozillazine article compacting folders reduces the size of the files used to hold email and email information. What happens is when an email is “deleted” it is actually hidden from the user but remains in file and is “marked for deletion”. Moving files also does the same thing. It creates a copy of the email in the respective “folder” in Mozilla Thunderbird and hides the original email in the previous folder. This means the size of the files related to email gets bigger and bigger.
The reasoning for this behaviour is reasonable. If a user deletes or moves emails in quick succession it makes it quicker to hide something than it is to physically remove / update a file on a hard disk. All email programs does this. The difference between Thunderbird and other email clients is that Thunderbird does not have automatic folder compacting set to run automatically.
If the folder you wish to compact have a lot of emails or have not been compacted before then it is best to back up your profile before doing so.
Whilst it’s good practice to have a backup compacting folders may take a while to complete and if anything wrong occurs it may corrupt the folder. The profile to back up is found in
Tools > Account Settings...
Select Local Folders in the left hand side and the profile path can be found in Local directory field.
If multiple local accounts are used each one should be backed up before bulk compacting.
To do all the folders the above steps needs to be performed on all folders or set up automatic compacting.
Mozilla Thunderbird supports automatic compacting based on size of a folder.