One of the lads here in work has got both his work mail and his NTL (*@upcmail.ie) set up in Outlook. He can send mails perfect from his work address, but when sending mails from his upcmail, 99% of them return a 'System Administrator: Undeliverable' Error. The error mail contains the following: 'The following recipients cannot be reached: 'email@example.com' on 22/04/2008 15:28 550 relaying mail to gmail.com is not allowed.
This happens regardless of the domain I send it to, or if the 'my server requires authentication' box is ticked in the account settings. It's Outlook 2007 he's using, and the work mail is IMAP. The annoying thing is that if I change the settings, like untick or tick the aforementioned box, sometimes a mail will send fine and then two minutes later it won't send again.