It sounds to me like Optus are offering their usual sterling service in the area of support and security. Port 923 does not sound like any of the default mail ports in use anywhere. Generally IMAP mail is received on Port 993 and send mail using SMTP on port 465 I would like to confirm those things on the Optus web site, but unfortunately if it is there it is well hidden. The est I could find was this https://www.optus.com.au/for-you/support/answer?id=1434 which at least confirms the server names to be used. in the form of Incoming mail server: mail.optusnet.com.au Outgoing mail server: mail.optusnet.com.au Username: Your OptusNet username (without @optusnet.com.au) Password: Your OptusNet email password So I have trawled the web and come up with these settings which appear fairly consistent accross the web forums and support sites I looked at. IMAP Server mail.optusnet.com.au Port 993 Security Policy Use SSL/TLS Username your-email@optusnet.com.au Password Your optusnet.com.au password SMTP Server settings SMTP Server mail.optusnet.com.au Port 465 Security Use SSL/TLS SMTP Username your-email@optusnet.com.au SMTP Password Your optusnet.com.au password So I really do not know as Optus appear to think security through obscurity will prevent the next hacking event. There is also the possibility that the issue is your antivirus. There have been a lot of those type things reported in the recent past. Check that there is not some sort of error window hidden behind other windows complaining about a certificate.