Exchange 2010 SP3 - "500 5.3.3 Unrecognized command" in SMTP Receive Log. Senders get message with the same problem

Question

Hi
My exchange environment consist with "Server Exchange 2010 SP3" and "Edge Server with Exchange 2010 SP3".
On Edge server in SMTP receive log i get every now and then an entry: xxxxxx 500 5.3.3 Unrecognized command.
It is serius problem because I got few e-mails from senders that they had problem with sending e-mail messages to our e-mail domain. They receive e-mail report, for example
Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the server for the recipient domain domena.plby
mail.domena.pl.
[95.xxx.xxx.xx].
The error that the other server returned was:
500 5.3.3 Unrecognized command
Could you help me to solve this problem? What could cause this problem?
Thank you in advance
Tomasz
Kind Regards Tomasz

Answer 1

I have the same issue, but there is plenty of hard disk storage on the Exchange Server. This seems to only be happening from Google Business account/ Postini. If I send message from my personal Gmail account it works fine, but an agency who is on Google
business seems to have issues sending to us and they are delayed, sometimes they get through and other times they just don't. 
Results from Mail Acceptance Test:
I enter my internal address and the remote users postini account as the test setup I received two emails from the tool but the end results of the test are:
 Error submitting mail
        Mail submission failed: Error message: Syntax error, command unrecognized. The server response was: 5.3.3 Unrecognized command.
Google support stated this:
Here are the logs from Postini trying to connect to mydomain.com (IP x.x.x.x) :
1. 2014/08/09 03:26:28 IP:209.85.218.50 SMTP-STLS:OK <null> TLSv1:RC4-SHA Validate:None 2. 2014/08/09 03:26:28 IP:209.85.218.50 SMTP LOG: Sender => FFD:MAIL FROM:<[email protected]>
3. 2014/08/09 03:26:28 IP:209.85.218.50 Fdal:Customer has null root_iid outbound default at org 100000001 4, 2014/08/09 03:26:28 IP:209.85.218.50 SMTP LOG: Sender <= FFD:250 Ok 5. 2014/08/09 03:26:28 IP:209.85.218.50 SMTP LOG: Sender => FFD:RCPT TO:[email protected]>
6. 2014/08/09 03:26:34 IP:209.85.218.50 To:[email protected] SMTP LOG: FFD => mydomain.com (x.x.x.x):Connection established 7. 2014/08/09 03:26:34 IP:209.85.218.50 To:[email protected] SMTP LOG: FFD <= mydomain.com (x.x.x.x):220 server.mydomain.com
ESMTP Service ready 8. 2014/08/09 03:26:34 IP:209.85.218.50 To:[email protected] SMTP LOG: Sender <= FFD:250 Ok 9. 2014/08/09 03:26:35 IP:209.85.218.50 To:[email protected] SMTP LOG: Sender => FFD:968 last bytes of data with dot 10. 2014/08/09 03:28:05
IP:209.85.218.50 To:[email protected] From:[email protected]:1119780 FP:2603ec6d38c69886eb1000e516fb745a0384e50b SMTP:Delivery still in progress after 90 seconds, recording FP 11. 2014/08/09 03:31:17 IP:209.85.218.50 To:[email protected] SMTP
LOG: FFD => mydomain.com (x.x.x.x):failed to send the last period(.) 12. 2014/08/09 03:31:17 IP:209.85.218.50 To:[email protected] oremote:Skipping Archive for Sender because did not deliver message 13. 2014/08/09 03:31:17 IP:209.85.218.50 To:[email protected]
SMTP:451 Remote connection lost - psmtp 14. 2014/08/09 03:31:17 IP:209.85.218.50 To:[email protected] SMTP LOG: Sender <= FFD:451 Remote connection lost - psmtp
As you can see, on line 9, at 03:26:35, we declared to the receiving server what we were about to send. At the line number 10, almost two minutes passed without an authorization (250 OK) from the receiving server. You can see our status being pushed back
at that moment to the receiving mail server: SMTP:Delivery still in progress after 90 seconds. Two more minutes later the connection was simply dropped and so we generated a 400 error message to encourage the sending server to try again later (since no bounce
was given per say from the receiving server, we won't hard-bounce it but defer it).
I hope this will help. you can try to look in the mydomain.com  inbound logs and see at that exact time what happened? Unfortunately, our visibility pretty much ends there as we don't have full visibility of your mail environment. You'll need to investigate
within your own environment unfortunately. If you have any more information to share with me, in case it might help me help you, feel free to do so. I'll leave this case opened for now.
Sincerely,
Patricia
Google Enterprise Support