How to configure a Sendmail Smart Host

Environment

• Red Hat Enterprise Linux 3
• Red Hat Enterprise Linux 4
• Red Hat Enterprise Linux 5
• Red Hat Enterprise Linux 6
• Red Hat Enterprise Linux 7
• Sendmail

Issue

• How to configure a Sendmail smart host
• How to forward all emails destined for remote recipients to a centralized mail server
• How to configure a relay host in Sendmail

Resolution

To configure a Sendmail Smart Host, follow these steps:

1. The /etc/mail/sendmail.mc file contains the following line:

   dnl define(`SMART_HOST',`smtp.your.provider')dnl
   

   Remove dnl ("delete through newline") from the beginning of the line :

   define(`SMART_HOST',`smtp.your.provider') dnl
   

2. Now substitute smtp.your.provider with the address of the SMTP relay server. There are four possible formats:

   • hostname - If MX record is present, it is used, otherwise DNS is looked up normally.

     For example:

     define(`SMART_HOST',`redhat.com')
     

   • [hostname] - DNS is looked up normally, MX record is ignored. Use this if the MX record is different from the hostname, and you do not want to use it.

     For example:

     define(`SMART_HOST',`[redhat.com]')
     

   • [IPv4 address] - The provided IPv4 address is used.

     For example:

     define(`SMART_HOST',`[192.0.2.4]')
     

   • [IPv6 address] - The provided IPv6 address is used.

     For example:

     define(`SMART_HOST',`[IPv6:2002:c0a8:51d2::23f4]')
     

     Note: if the sendmail system does not have a working DNS functionality, please see: Can sendmail be used without DNS?.

3. Install the sendmail-cf package:

   # yum install sendmail-cf
   

4. Generate a new sendmail.cf file. This happens automatically once the sendmail-cf package is installed. Note that any custom changes to sendmail.cf are lost in the process, so take a backup if necessary.

   For RHEL 6 and earlier:

   # service sendmail restart
   

   For RHEL 7 and later:

    # systemctl restart sendmail
   

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