See What does this error mean? for the reasons for some of the error codes. The error will resemble this, with a different error number and possibly a better explanation than 'unknown error' Browse to Tools, Send/Receive, Send/Receive settings, Show progress and click on the error tab. An antispam or antivirus scanner is scanning outgoing mailįirst, check the Send/Receive progress dialog for error codes.Using a program that accesses pst or ost data, such as Desktop Search, Lync, etc.Wrong password or not authenticating properly with the mail server.
If the message is not too large to send, other possible causes include:
It's most likely a read receipt that won't send. If you don't see a message in your outbox but Outlook is trying to send something, see Hidden Stuck Messages. If you are using a Comcast account and messages aren't sending, you need to change your outgoing server to use port 587 or 465 and set Use this following type of encrypted connection to None then click OK.
