Unable to send email from Outlook

[quasid.99]

there is a communication problem between Outlook and your outgoing mail server, so the email is stuck in Outbox because Outlook can't connect to your mail server to send it.

[jakosaleena99]

How do I fix an email sending problem in Outlook?
Although doing so can fix the issue, be aware that you will have to recreate any email message you were attempting to send.
Open the Send folder.
Delete all of the messages in the folder.
Quit Outlook.
Restart Outlook.
Try sending an email to see if the issue is resolved.

[quasid.99]

Can receive emails but not send in Outlook?
Cause: Some POP and IMAP email accounts use an outgoing mail (SMTP) server that requires authentication.

[katemith222]

Outlook 2007, 2010 and 2013 (and Outlook 2003 SP2 and above)
Go to Tools menu > Account settings.
Select the relevant GreenNet email account, and click the “Change” button (third from the left above that line)
Make sure that the “Outgoing mail server (SMTP)” box reads “smtp.gn.apc.org” if this is a GreenNet account.