From the GMail help pages:
"As a security measure to prevent potential viruses, Gmail doesn't allow you to send or receive executable files (such as files ending in .exe) that could contain damaging executable code.
Gmail won't accept these types of files even if they are sent in a zipped (.zip, .tar, .tgz, .taz, .z, .gz) format. If this type of message is sent to your Gmail account, it is bounced back to the sender automatically."
Funny thing is, my backup.tgz file did not contain any executables...only some files ending in .cmd !
Solution: Rename the backup.tgz file, problem solved!