I am using FileZilla to manage a remote host via SFTP and it connects fine when the username does not contain a @ symbol. For example, the username "administrator" will work but "email@example.com" will not work as cPanel suggests it should.
Further, firstname.lastname@example.org@ftp.example.com doesn't seem like proper syntax and thus the server is interpreting it wrong.
I had a glimpse at the FileZilla forums and someone described the same problem and asked if the @ should be escaped but the only answer to that was it should not be escaped, so it didn't really answer their question nor mine.
In cPanel there are a couple of users that use @ symbols in the usernames and one or two that don't.
Appreciate your input, cheers.