Usually Outlook will give this error along with an error code such as 0x80042109 because it's not able to connect to our mail server to send email via TCP/IP port 25. This is the standard port for SMTP mail communication but it's often abused by spammers. Some ISPs and almost all mobile ISPs block it other than for access to their own SMTP mail servers.
For this reason we also run SMTP on port 587.
To setup Outlook to do this
1) Open Outlook.
2) Tools -> Accounts
3) go to the "Mail" tab.
4) select the account and click "Properties".
5) Go to the "Advanced" Tab in the new window.
6) Set the "Outgoing mail (SMTP)" option to 587 from 25. Hit "Ok".
Outlook should now use port 587 for sending mail.
Alternatively, you could also set up the outgoing server to be your ISP's mail server.
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