I use a HTC Desire HD android mobile. My 8 GB SD card was getting full so I removed photos and videos that had already been copied to my PC. But I became a bit curious because of the 8 GB, the images and videos where not consuming the entire space.
To investigate further I connected the android phone using USB to my PC and used the PC:s explorer to dig into the SD-card.
I found out that the mail program stores images and other parts of emails in the Downloads folder on the SD card. After 1,5 years of use there where a significant amount of files there.
After deleting them i noticed there was a folder called .Mail (i.e. Downloads/.Mail) which contained >21.000 files consuming some 700 MB of space. The filenames began with "token". After some Googleing it seemed by the discussions I found that these are temporary files created each time a message is pushed to the phone. I decided to take a shot and delete them.
After deleting all files in the .Mail folder I opened the mail app on the phone. For a short moment a text was displayed "Preparing SD card". Then my inbox showed up and everything worked normal with the only difference that the mail app seemed a bit quicker now.
My conclusion is that it is probably safe to delete the content of the .Mail folder once in a while to free up some space.