Did your check your SPF record?
I'm running Proftpd. Sometimes I also use the commands "ftpwho" and ftptop". Hope that helps!
You are probably blacklisted. You can check here Changing you IP is a bit extreme... You could change your IP, keep in mind also to change...
I guess this could help you: http://olivier.sessink.nl/jailkit/howtos_sftp_scp_only.html
What FTP server are you using?
Separate names with a comma.