Solved: cannot telnet to mail server on port 25

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Windows Server 2003

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mrosier🇺🇸

cannot telnet to mail server on port 25

Hello Folks,

I have a mail server using just the IIS 6.0 SMTP relay. I am trying to mass mail to my customers on comcast, and my mail is not leaving the outgoing queue on it. I have disabled my antivirus. I cannot telnet to mx1.comcast.net on port 25, BUT I can telnet to its IP address. This is not the case with other mail servers like yahoo's. I am not having this trouble on my laptop as I can telnet to mx1.comcast.net and mx2.comcast.net. But I can only use those IPs to telnet to on the SMTP server. Is this possibly keeping this mail in the outgoing queue? I have tried rebooting and restarting the SMTP service. It is an update 2003 Windows server. Thanks!

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Ilya Rubinshteyn🇺🇸

It may be using secured SMTP for outbound transfer, I believe the port is 465? check the settings for comcast mail transfer on their site. You also need to have an authentication mechanism for the site.

mrosier🇺🇸

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Well I was sending ok over the weekend without issue, and I actually have clear status on my connection sending the mail. They have confirmed to me that I should have a clear path to deliver my mails to comcast.net users. I have authentication requirements on my SMTP service on that server as well. What I don't understand is why I can telnet to port 25 of the IP addresses of those two servers but not the regular canoical names of those two servers? Could that be stopping me somehow?

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Ilya Rubinshteyn🇺🇸

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mrosier🇺🇸

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DNS did the trick! my primary DNS server under the TCP/IP settings was out of date, and I forgot to update it as I have that machine's TCP/IP settings manually configured as it is a mail server. I changed it to the current DNS primary, was able to make the telnet sessions go with the FQDNs, flushed DNS, restarted the SMTP service, and mail started flowing. Thanks so much!

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ExchangeOnTheRocks🇮🇳

From the IIS box, type nslookup and try to resolve the MX records of comcast.net. Are you able to see all the details successfully?
Secondly, on the properties of the SMTP virtual server, do you have any DNS server specified under Advanced settings?

Adam Brown🇺🇸

It may be using secured SMTP for outbound transfer, I believe the port is 465?

As a note, Secured SMTP does not use a different port by default. Secure SMTP uses the STARTTLS command in SMTP to initiate a TLS secured SMTP communication. You can assign a different port for Secure SMTP from client to server (Like Google does, primarily to bypass ISPs that block SMTP for residential users), but most secure SMTP for server-to-server still goes over port 25.

Gaurav Singh🇮🇳

please check with your provide if the SMTP ports are opened from their end, its likely b the issue with port blocking

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mrosier🇺🇸

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The DNS check reminder showed me I had an out of date DNS server on the TCP/IP settings of that network connection. Thanks!

Windows Server 2003

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Windows Server 2003 was based on Windows XP and was released in four editions: Web, Standard, Enterprise and Datacenter. It also had derivative versions for clusters, storage and Microsoft’s Small Business Server. Important upgrades included integrating Internet Information Services (IIS), improvements to Active Directory (AD) and Group Policy (GP), and the migration to Automated System Recovery (ASR).