Are you using the System.Net.Mail to send File Attachments? Is the file attachment above 3MB?
If yes; you have just found out the reason for the error. The System.Net.Mail Class throws an exception if a file attachment is over 3MB
Microsoft has a patch available which solves this issue
If you have any issues downloading the file; you may post your comment on this post.
Once the patch is applied; you should now be able to send large attachments using your Powershell Send-MailMessage cmdlet.
Please ensure that you use the -UseSSL in your cmdlet. If you leave out -UseSSL; you are most likely to encounter the below error
Send-MailMessage : The operation has timed out.