Can anyone tell me what this actually means: #< #5.0.0 smtp; 510 Did not receive the expected protocol response.> #SMTP#
I keep seeing this attached to "Delivery has failed to recipients of distribution list:" message that hits my administrator mailbox. The message is being generated by one of my Ironmail server.
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.
It the problem you are seeing is the size limit issue, it was fixed with 6.7.2 HF6. But the size restriction is only one of many things that can cause the error.