Seems that there's a lot of people out there that do not know how to copy a file with a colon in it's name (for example a:a.txt) using scp or rsync, so here's an explanation:
The trick is to know where you're using the : character. If it's a remote file there's no problem and no need of escaping since you preceded already by an :. Example:
scp email@example.com:a:a.txt .
If it's a local file you just have to say it explicitly by expressing a path. Example:
scp ./a:a.txt firstname.lastname@example.org:
This works for me at least on OpenSSH 4.3.