Configure sendmail as a smart host

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Smart host is very handy if you are on dial up network or sometimes a host finds mail that it is unable to deliver directly to the desired remote host.

In large network, it is good idea to have a single host/mail server acting as MTA. Smart hosts are usually used when all other methods of delivery have failed. In the case of the organization with the private network, it would be perfectly reasonable to have the hosts attempt to deliver mail directly first, and if that fails then to send it to the smart host. This relieves the smart host of a lot of traffic because other hosts can directly send mail to other hosts on the private network.

The SMART_HOST macro allows you to specify the host that should relay all outgoing mail that you are unable to deliver directly, and the mail transport protocol to use to talk to it.

Open your configuration file:# vi /etc/mail/sendmail.mcAppend or modify macro that read as follows :define(`SMART_HOST',`')Replace with your actual smtp server address. If line contains word, dnl remove the dnl word. Regenerate a new config file with m4 command:# m4 /etc/mail/ > /etc/mail/sendmail.cfRestart sendmail service:# /etc/init.d/sendmail restart

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Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin, DevOps engineer, and a trainer for the Linux operating system/Unix shell scripting. Get the latest tutorials on SysAdmin, Linux/Unix and open source topics via RSS/XML feed or weekly email newsletter.

3 comment

  1. I can’t send emails (sendmail) to other domains in the internet. I can send mails to but to some domains it bounce back, giving me a message of unknown host.

    How can I correct this?

  2. How to do conditional SMART HOSTING based on FROM header ?

    For example, I set from address in an email of mine to my gmail address, then it should go via Gmail’s SMTP with my *GMAIL* account; otherwise it should go via my Google Apps account.

  3. This is not about configuring sendmail AS a smart host, it’s about configuring sendmail to USE a smart host. The title suggests setting up a server to act AS a smart host rather than just how to relay to an existing one.

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