System Administration

sendmail only sending to gmail

Valued Contributor

sendmail only sending to gmail



My sendmail has stopped sending mail to my local domain but it is still able to send mail to gmail.


I am using RHEL 5 (32-bit).


I am getting the following error in the maillog when I attempt to send anything to my local domain:


DSN: Service unavailable


I am using an external SMTP and can telnet over port 25, my DNS is also fine as this has been working until only recently with no changes (DNS) made locally. I have been in touch with the ESMTP relay and apparently no changes have been made here either?


Any ideas on how to resolve this?




Honored Contributor

Re: sendmail only sending to gmail

Step 1.)

Read the mail log (/var/log/mail.log in RHEL5, like most Linuxes). There should be multiple messages regarding each processed email: at least one when Sendmail receives the mail from userspace for processing, and then another when attempting to relay it onwards. Pay attention to where it's sending the message, and what's the result of the delivery attempt.


Step 2.)

Does the result of step 1 make sense? In other words, is Sendmail trying to push the email to the correct server? It might be that this system has no Sendmail "smarthost" setting configured, and as a result, Sendmail is trying to follow the DNS MX records and is trying to relay to an unexpected destination. Perhaps firewall configuration has been changed to allow the internal hosts to use only the "internal" mail server, instead of the "incoming external" one (with all the anti-spam, anti-scam and anti-malware features you need to protect your users today).


A Delivery Status Notification "Service unavailable" could mean the connection was not successful at the TCP level: this might be a network firewall blocking the connection, or a server rejecting the connection based on IP addresses (perhaps using a software firewall). Or it might be an actual network failure...


Step 3.)

When you know the IP address of the relay destination and the accurate time of failed relay attempt(s), you can ask the administrator of the other server, e.g. "At XX:YY:ZZ my server (IP a.b.c.d) tried to relay a mail to your server, but it seems your server did not like it. Could you please check your logs and tell me what exactly happened on your side?" If the answer is along the lines of "Strange, there is absolutely no trace of a connection attempt from your server's IP at that time in my logs", then it's definitely time to talk with the network/firewall admins, and now you have some hard evidence to show them.