28.6. Advanced Topics

This section covers more involved topics such as mail configuration and setting up mail for an entire domain.

28.6.1. Basic Configuration

Out of the box, one can send email to external hosts as long as /etc/resolv.conf is configured or the network has access to a configured DNS server. To have email delivered to the MTA on the FreeBSD host, do one of the following:

  • Run a DNS server for the domain.

  • Get mail delivered directly to to the FQDN for the machine.

In order to have mail delivered directly to a host, it must have a permanent static IP address, not a dynamic IP address. If the system is behind a firewall, it must be configured to allow SMTP traffic. To receive mail directly at a host, one of these two must be configured:

  • Make sure that the lowest-numbered MX record in DNS points to the host's static IP address.

  • Make sure there is no MX entry in the DNS for the host.

Either of the above will allow mail to be received directly at the host.

Try this:

# hostname
example.FreeBSD.org
# host example.FreeBSD.org
example.FreeBSD.org has address 204.216.27.XX

In this example, mail sent directly to should work without problems, assuming Sendmail is running correctly on example.FreeBSD.org.

For this example:

# host example.FreeBSD.org
example.FreeBSD.org has address 204.216.27.XX
example.FreeBSD.org mail is handled (pri=10) by hub.FreeBSD.org

All mail sent to example.FreeBSD.org will be collected on hub under the same username instead of being sent directly to your host.

The above information is handled by the DNS server. The DNS record that carries mail routing information is the MX entry. If no MX record exists, mail will be delivered directly to the host by way of its IP address.

The MX entry for freefall.FreeBSD.org at one time looked like this:

freefall		MX	30	mail.crl.net
freefall		MX	40	agora.rdrop.com
freefall		MX	10	freefall.FreeBSD.org
freefall		MX	20	who.cdrom.com

freefall had many MX entries. The lowest MX number is the host that receives mail directly, if available. If it is not accessible for some reason, the next lower-numbered host will accept messages temporarily, and pass it along when a lower-numbered host becomes available.

Alternate MX sites should have separate Internet connections in order to be most useful. Your ISP can provide this service.

28.6.2. Mail for a Domain

When configuring a MTA for a network, any mail sent to hosts in its domain should be diverted to the MTA so that users can receive their mail on the master mail server.

To make life easiest, a user account with the same username should exist on both the MTA and the system with the MUA. Use adduser(8) to create the user accounts.

The MTA must be the designated mail exchanger for each workstation on the network. This is done in theDNS configuration with an MX record:

example.FreeBSD.org	A	204.216.27.XX		; Workstation
			MX	10 hub.FreeBSD.org	; Mailhost

This will redirect mail for the workstation to the MTA no matter where the A record points. The mail is sent to the MX host.

This must be configured on a DNS server. If the network does not run its own DNS server, talk to the ISP or DNS provider.

The following is an example of virtual email hosting. Consider a customer with the domain customer1.org, where all the mail for customer1.org should be sent to mail.myhost.com. The DNS entry should look like this:

customer1.org		MX	10	mail.myhost.com

An A> record is not needed for customer1.org in order to only handle email for that domain. However, running ping against customer1.org will not work unless an A record exists for it.

Tell the MTA which domains and/or hostnames it should accept mail for. Either of the following will work for Sendmail:

  • Add the hosts to /etc/mail/local-host-names when using the FEATURE(use_cw_file). For versions of Sendmail earlier than 8.10, edit /etc/sendmail.cw instead.

  • Add a Cwyour.host.com line to /etc/sendmail.cf. For Sendmail 8.10 or higher, add that line to /etc/mail/sendmail.cf.

All FreeBSD documents are available for download at http://ftp.FreeBSD.org/pub/FreeBSD/doc/

Questions that are not answered by the documentation may be sent to <freebsd-questions@FreeBSD.org>.
Send questions about this document to <freebsd-doc@FreeBSD.org>.