I have a customer who sometimes has connectivty problems with his Outlook. Sometimes he will just start receiving 0x8004210B error messages when he tries to do a Send/Receive (SMTP, outgoing related).
Windows Firewall is turned off and I have tried to disable Symantec Endpoint as well to see if it could be causing it. I can however send a email via telnet if I telnet from his PC to the SMTP server. So there doesnt appear to be a problem with the mail server or on his PC. It seems to be related to Outlook only.
Another user has this problem as well, however not as frequently. This seems to have started at the same time as no one has had this problem previously. Maybe caused by a Windows/Office update?