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I have installed exim4, and my /etc/hosts file looks like this:

127.0.0.1     localhost

127.0.1.1     mycomputer

192.168.100.5 rpi.mydomain.com

When I run exim -bt john@rpi.mydomain.com to test deliverability, it says:

R: dnslookup for john@rpi.mydomain.com

john@rpi.mydomain.com is undeliverable

Why doesn't it use the /etc/hosts file?

Additional information:

• System: Ubuntu.


• The address in the /etc/hosts file is valid. I verified this using telnet rpi.mydomain.com 25.

/etc/nsswitch.conf:

passwd:         compat systemd

group:          compat systemd

shadow:         compat

gshadow:        files



hosts:          files mdns4_minimal [NOTFOUND=return] dns myhostname

networks:       files



protocols:      db files

services:       db files

ethers:         db files

rpc:            db files



netgroup:       nis

Please see this part of Exim documentation.

In short, email delivery relies pretty heavily on MX records of the DNS system, and there is no equivalent for them in /etc/hosts.

Apparently Exim's test feature first takes into account whether Exim is configured to use a smarthost or not; if it has, it would report the address(es) of the smarthost it knows about. (Smarthost configuration = old sendmail-based email terminology meaning "don't attempt to deliver directly to destination address - instead, send all outgoing email to one specific server for further processing.")

If no smarthost configuration is in use, the test checks for the presence of a DNS MX record for rpi.mydomain.com. If the MX record is of the following form, it is interpreted to mean explicitly "no email service for this domain" and the search ends:

rpi.mydomain.com. IN MX 0 .

If there is no MX record in DNS and rpi.mydomain.com is listed in Exim configuration item mx_domains, the search will also end with a "mail is undeliverable" error.

If the above did not already terminate the search, Exim will check for A or AAAA records in the DNS.

In your own network, if you wish to use rpi.mydomain.com as your mail server, you should configure your local Exim to use it as a smarthost. Here's the relevant bit of Exim documentation. In short, replace the default dnslookup mail router configuration block with this Exim configuration block:

send_to_smart_host:

  driver = manualroute

  route_list = !+local_domains rpi.mydomain.com

  transport = remote_smtp

This should cause all mail whose destination is not the local inbox /var/mail/<username> to be sent to rpi.mydomain.com for further processing.

Well-behaving mail servers have long been expected to have a valid DNS registration, and also a valid reverse DNS registration that is in agreement with the forward DNS record.

Generally, when a mail server receives an incoming connection that is not authenticated and does not come from a known-trusted network, the mail server will typically first make a reverse DNS lookup to find out the name of the system trying to connect. Then it makes a forward DNS lookup using that name, and compares the resulting IP address(es) to the actual source IP of the connection. If there is no match, or there is no reverse DNS record, the connection is assumed to come from a possible spammer and can be subjected to stricter checks or outright disconnected. This is one of the oldest anti-spam checks.