Same problem here - solved (for now, at least, for me...)
I had the "Low on Space" warning, and found that the mail app was using a massive 770MB!!
I deleted current mail and checked it had gone from trash also - next-to-no difference.
I deleted the email account - no difference.
After lots of searching....
Following these instructions for TEMP ROOT here:
I gained temporary root access, and tried deleting large number of files in the files in /data/data/com.htc.android.mail/ (which appeared to be the attachments of emails deleted long ago). This made a small, but still not significant difference. I searched through the file system some more but couldn't find where all this memory was being used.
What worked for me, once I had temp root, was the pm command. At the # commend line of temp root, type pm for an explanation of its usage.
I used the command:
pm clear com.htc.android.mail
to "delete all data associated with the package"... which reduced mail's usage from 700MB+ to 64kB :-)
(Obviously, if I hadn't already deleted it, my email account and mail would have been swept away also...)
Now... to more carefully setup my email account again...