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John Atkinson
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Exchange server cannot send email to a particular email server

I'll start with the question:  how can I fix my Exchange server's new inability to connect to a remote mail server, to send mail?  Now, the background...

My client's on-premise Microsoft Exchange 2010 server recently lost the ability to send mail to accounts at a domain I'll call pulp.com.  I'm trying to restore this capability.  Specifically, when my client sends an email to, say, john@pulp.com, the message appears to have been sent, but some time later, my client receives a non-delivery report saying

400 4.4.7 Message delayed

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My client receives several of these, spaced out by hours, as the Exchange server retries sending the email.  Eventually, an NDR comes in stating

#550 4.4.7 QUEUE.Expired; message expired ##

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pulp.com is fine.  (The owner of pulp.com is also a client of mine, as it happens.)  Messages are arriving just fine to john@pulp.com from other senders.  Only my Exchange client is having trouble.

I checked Exchange 2010's SMTP logs for clues.  Here, I found scores of entries that resemble these:  (Note that I'm substituting for pulp.com's real IP address)
2018-10-31T23:19:47.336Z	0	*	attempting to connect 
2018-10-31T23:20:08.338Z	1	*	Failed to connect. Error Code: 10060, Error Message: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond

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So, there's a connection attempt, and 20 minutes later, an acknowledgement that it didn't work, due to the remote server not responding.
The SMTP log shows hundreds of successful connection attempts, and message transmissions, all beginning with an attempt to connect on port 25.  Only pulp.com's IP address shows up in these "Failed to connect" scenarios.

The connection only started failing a few days ago.  20 hours before the first failure was the most recent successful connection attempt.  A successful attempt shows up in the log like this:
 Date       Time          Seq. #   LocalEndpoint    RemoteEndpoint
 2018-10-31T03:43:59.225Z  0                  * attempting to connect 
 2018-10-31T03:43:59.445Z  2 < 220-mail.pulp.com ESMTP Exim 4.91 #1 Tue, 30 Oct 2018 20:43:59 -0700 
 2018-10-31T03:43:59.445Z  3 < 220-We do not authorize the use of this system to transport unsolicited, 
 2018-10-31T03:43:59.445Z  4 < 220 and/or bulk e-mail. 
 2018-10-31T03:43:59.445Z  5 > EHLO mail.onPremExchangeClient.net 
 2018-10-31T03:43:59.506Z  6 < 250-mail.pulp.com Hello mail.onPremExchangeClient.net [] 
 2018-10-31T03:43:59.506Z  7 < 250-SIZE 52428800 
 2018-10-31T03:43:59.506Z  8 < 250-8BITMIME 
 2018-10-31T03:43:59.506Z  9 < 250-PIPELINING 
 2018-10-31T03:43:59.506Z  10 < 250-AUTH PLAIN LOGIN 
 2018-10-31T03:43:59.506Z  11 < 250-STARTTLS 
 2018-10-31T03:43:59.506Z  12 < 250 HELP 
 2018-10-31T03:43:59.506Z  13 * 385863 sending message 
 2018-10-31T03:43:59.506Z  14 > MAIL FROM:<client@onPremExchangeClient.net> SIZE=7949 
 2018-10-31T03:43:59.506Z  15 > RCPT TO:<john@pulp.com> 
 2018-10-31T03:43:59.568Z  16 < 250 OK 
 2018-10-31T03:43:59.568Z  17 < 250 Accepted 
 2018-10-31T03:43:59.568Z  18 > DATA 
 2018-10-31T03:43:59.629Z  19 < 354 Enter message, ending with "." on a line by itself 
 2018-10-31T03:43:59.761Z  20 < 250 OK id=1gHhQF-007GaQ-Dn 
 2018-10-31T03:43:59.761Z  21 > QUIT 
 2018-10-31T03:43:59.823Z  22 < 221 mail.pulp.com closing connection 
 2018-10-31T03:43:59.823Z  23 - Local 

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Just to sanity check, I tried telnet on the Exchange server:
C:\>telnet mail.pulp.com 25 
Connecting To mail.pulp.com...Could not open connection to the host, on port 25: Connect failed

C:\>telnet 25 
Connecting To not open connection to the host, on port 25: Connect failed

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From any offsite location, telnet to these IPs connects fine.   Sending mail to anyaddress@pulp.com works from any other domain I've tried.  

How do I figure out why neither Exchange server's SMTP service, nor telnet, can connect to this mail server, when everyone else can?

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Jackson Favre

8/22/2022 - Mon