Sendmail Mailertable

Mailertable (/etc/mail/mailertable)

Routing information for external delivery. Functionally, these are like the SMTP Connectors within Exchange. The mailertable entries can override everything including smarthost definitions. This is required for internal mail routing – our sendmail servers should not transmit email for @windstream.com to the MX records but rather the destination we intend. We also use mailertable entries to force B2B communication over internal secured channels.

If a server is unable to deliver mail to a specific domain (e.g. one of our public IP addresses gets blacklisted), a mailertable entry can be used to direct all mail destined for the domain through one of our servers still able to make delivery.

The file contains two columns, domains and actions. Domains can be ends-with substring matches:
.anythingfromthisdomain.com

Will match @thishost.anythingfromthisdomain.com as well as @thathost.anythingfromthisdomain.com. Domains can also be a full match of the right-hand side of the email address:
justthisemaildomain.com

Which will match @justthisemaildomain.com. The most “accurate” match will win, not just the first match in line. So if your file contains the following:
.mysampledomain.com relay:[10.10.10.10]
thishost.mysampledomain.com relay:[20.20.20.20]

Mail destined for thishost.mysampledomain.com will be sent to 20.20.20.20

Actions contain both a mailer and a host. The mailer can redirect messages to local users:
.egforlocaldelivery.com local:username

Or it can force an error response:
Baddomain.com error:5.3.0:Unknown User

Our use of the mailertable, though, is to redirect mail destined for the domain:
windstream.com relay:[twnexchinbound.windstream.com]
newacquisition.com relay:[theirinternalhost.theirdomain.com]

In these cases, the square brackets around the destination override the MX record. To reroute a domain’s delivery destination, then, it is imperative that the host be enclosed in square brackets.

To commit changes to the file, either use “make” from within /etc/mail to commit all changes or the following command to commit just the changes to mailertable:
makemap hash /etc/mail/mailertable < /etc/mail/mailertable

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