Undeliverable return email with multiple errors- Resolution with screenshots

Situation: The client is running exchange 2010. Recently, some of users get many Undeliverable return emails. here are some return messages:

Postini rejected your message to the following e-mail addresses:
Postini gave this error: Syntax error, unrecognized command - psmtp
yahoo.com rejected your message with this error: Command Unimplemented
Delivery is delayed to these recipients or groups
The server will keep trying to deliver this message for the next 1 days, 19 hours and 45 minutes. You'll be notified if the message can't be delivered by that time.
secureserver.net gave this error: #5.5.1 command not recognized

Troubleshooting: 1. We analyzed the log and found this log:

-13T13:53:55.986Z,Exchange 2010 Outbound SMTP Mail,08D026504B789098,2,10.20.20.216:42791,72.167.238.201:25,<,220 p3pismtp01-064.prod.phx3.secureserver.net ESMTP,
2013-06-13T13:53:55.986Z,Exchange 2010 Outbound SMTP Mail,08D026504B789098,3,10.20.20.216:42791,72.167.238.201:25,>,EHLO EXCH-CAS02.chicagotech.net,
2013-06-13T13:53:56.064Z,Exchange 2010 Outbound SMTP Mail,08D026504B789098,4,10.20.20.216:42791,72.167.238.201:25,<,250-p3pismtp01-064.prod.phx3.secureserver.net,
2013-06-13T13:53:56.064Z,Exchange 2010 Outbound SMTP Mail,08D026504B789098,5,10.20.20.216:42791,72.167.238.201:25,<,250 SIZE 31457280,
2013-06-13T13:53:56.064Z,Exchange 2010 Outbound SMTP Mail,08D026504B789098,6,10.20.20.216:42791,72.167.238.201:25,*,321699,sending message
2013-06-13T13:53:56.064Z,Exchange 2010 Outbound SMTP Mail,08D026504B789098,7,10.20.20.216:42791,72.167.238.201:25,>,MAIL FROM:<jbrown@chicagotech.net> SIZE=5581,

2. We also test it using telnet

telnet bully.com 25
220 Postini ESMTP 200 y674_pstn_c4 ready. CA Business and Professions Code Sect
ion 17538.45 forbids use of this system for unsolicited electronic mail advertisements.
EHLO EXCH-CAS02.chicagotech.net
250-Postini says hello back
250-STARTTLS
250-8BITMIME
250 HELP
MAIL FROM:<claw@chicagotech.net>
250 Ok
RCPT TO:<csmith@bulley.com>
250 Ok
data
354 Feed me
test
.
571 Message Refused

It seems to us the Exchange did send the email but there is firewall or email filter reject the email.

Suggestions: 1. It could be DNS issue. Try to use External DNS on Exchange. Please refer to this page: How to setup Exchange 2010 using External DNS Lookups - Step by step with screenshots.

2. Based on Microsoft article,  "These issues may occur when both the following conditions are true:
• The Exchange server is located behind a Cisco PIX firewall device that has the Mailguard feature enabled.

• The Auth and Auth login Extended Simple Mail Transfer Protocol (ESMTP) commands are removed by the firewall.

Please refer to this page to turn off mailguard: How to turn off Mailguard feature in Cisco ASA Firewall.

3. In our case, we found the client has modusgate from Vircom to reject the emails. We follow this how to pass through the modusgae: How to configure bypass in Virmail/modusGate - Step by step with screenshots

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