If you are working in
.NET framework 4.0
and want to send a mail with an attachment size greater that
, then your might need to install a Microsoft .NET framework 4 Client Profile patch.For detail and download, take a look at this
I see. Then here are things to check
1. Firewall is allowing to send mail through the mail server.
2. Network issue.
3. The mail server is properly working.You can check this by sending mail using
I hope this will help you well.