I am having a similar, if not the same, issue. When I set up my POP3 account info, i get the message "Invalid login "
I've tried both with and without the @ symbol in the user name. I can login to this box with no problem using webmail and other methods, just not with osTicket (1.6 RC3)
If a ticket is created with the online form, an alert and notiification email are sent.