Originally Posted by amanda_lakai

There can be various possible reasons for this problem:
•Your message size is too large.
•Some Outlook add-in
•Authentication with the mail server is not done properly or mail server is offline.
•Default mail account has not been set
•Antivirus or firewall installed in your system
•Installed programs(such asesktop Search) that accesses the Outlook data files
•An antispam or antivirus scan is running on outgoing mails.

The most probable reason is the size of your message to be sent or the attachment is of huge size. In such case reduce the size and try re sending. I hope this will fix the issue.