Microsoft Outlook has difficulty using POP-before-SMTP when trying to relay mail through a server.
There is no method in Outlook to tell it to check a POP account before trying to send mail via SMTP. The simplest workaround is to simply click the Send button a second time. As it has checked the pop account the first time, the SMTP mail will be sent successfully the second time.
Outlook also supports authenticated smtp which does not require logging into POP but rather into SMTP, simply tell Outlook that your SMTP server requires authentication. Then in GMS enable the "Allow AUTH" option under the Incoming > ESMTP page.
A slightly clumsier workaround within Outlook is to set up a second account with exactly the same details as your main account but calling it "aaaa", select the option "Include this account when receiving mail or synchronizing" but make sure not to make it your default account. Make sure it is higher in your list of accounts than the default account. This should ensure that you POP mail prior to sending.
Keywords:Relay, Outlook, POP-before-SMTP