It most likely is caused by the message size. This is a known issue with IronMail where it does not properly handle or report the "Message Size Exceeded" error as a permanent failure.
You can check the maximum size the recipient server accepts by doing a simple telnet to their server and then typing EHLO mydom.com:
telnet <ip address/hostname of recipient server> 25
telnet mail.mydom.com 25
Connected to mail.mydom.com.
Escape character is '^]'.
220 SMTP Proxy Server Ready
250-ESMTP Server Ready
If this isnt the case, then we would need to see a packet capture to determine what invalid response we are seeing.