I'm a big user of the Thunderbird e-mail program.
One big problem... it doesn't store or forward DXF attachments correctly. A big pain.
If anyone else has had this problem, maybe we can all chip in and get a programmer to put a fix in?
I am not a thunderbird user, but have had problems many times trying to transfer several types of files natively.
My solution is to ALWAYS zip them up first and have never had a problem that way!
Sometimes I don't think it is the e-mail program, but rather one of the ISPs or a point in between.
AKA Country Bubba (Older Than Dirt)