I don't use Apple Mail but in ~/Library/Mail, there are lots of files and folders (gigabytes worth). What is this from ? More importantly, can I delete them ? I use Thunderbird for mail and in ~/Library/Thunderbird/Profiles.... I have an Imap folder with what is I presume my current mail data. This "data" was created a couple of years ago and was last modified minutes ago. Makes sense, of course. The folder above, the one related to Apple Mail, was created within a few days of me getting this machine (and setting up Mail). It was last modified about a month ago. Not sure why obviously. I haven't even opened the Mail app in a couple of years.