FTP Problem

[Wintermute]

I can ftp fine to all my domains- at home i have verizon DSL.

Some of my customers have Comcast and they can not connect via ftp. What Gives ?

They can connect to OTHER FTP servers on other hosts just not mine. so its not their firewall...i am not blocking on my side either..

They have all the correct settings (pasv diabled etc).

Any ideas ?

thanks