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Could not resolve mail server name because DNS server did not respond in time.

No inbound email even you can send emails

Can't past SMTP test because of modusGate or Vircom

Situations: 1. The client is running Exchange 2010 with Microsoft failover cluster and there are two CAS servers. They can send email but can't receive any outside email. The internal email works fine.

2. When outside people send an email to your email address, they may receive this message: Could not resolve mail server name because DNS server did not respond in time.

3. When you test SMTP using telnet, the prot 25 is open, but you can't continue the rest test as shown below.

220 mai.chicagotech.net modusGate ESMTP Receiver Version 5.1.1024.13396
Ready
EHLO gmail.com
250-mai.chicagotech.ne
250-SIZE 31457280
250-ETRN
250-ENHANCEDSTATUSCODES
250-X-IMS 5 -1
250-DSN
250-VRFY
250-AUTH LOGIN
250-AUTH=LOGIN
250 8BITMIME
MAIL FROM:chicagotech@gmail.com
501 5.5.2 Syntax error in address chicagotech@gmail.com

Troubleshooting: 1. As said, telent port 25OK.

2. nslookup shows correct MX record.

3. Finally, we found there are a lot email stay queue in Vircom/modusGate and waiting for delivery. When we checked the Connection, we found the Route IP for CAS is 10.0.5.220. We could not ping the IP adders because the failover cluster broken last week and the failover doesn't work.  The temporary solution is modify the Routeon modusGate to ping to one CAS IP only. After that, the queue emails released from the modusGate.

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