Error in sending mail after submit or save

when I save attendance Strength and leave request .Mail is not sent .when i check email queue ,It goes in not sent .this error is happening for past 2 weeks

Have you checked all other email is going the properly and also check the error log

None of the mail is being sent .and I also checked the error log . SMPT error frappe.exceptions.ValidationError: Invalid Outgoing Mail Server or Port: [Errno 101] Network is unreachable

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The problem is at the network level or SMTP server then. You’ll need to do your troubleshooting there. Routing, IP addressing, making sure your SMTP server is even running and listening on port 25, etc., etc.