Recipient Verification With Exim As A Forwarding MX Host
If you use Exim as an forwarding MX hosts (secondary MX or mail hub) for some internal Mail-Systems like Lotus Domino or Exchange, you want to set up Exim in a way, that it verifies that the recipient of an email exists before Exim accepts the mail for delivery/forwarding.
One way to do so, would be to setup LDAP on the recipient system and do a lookup against that. But there is an easier way to do that – Callout verification.
All you have to do is create a „local_rcpt_callout” file in your Exim configuration directory (usually /etc/exim) and populate it with the domain names (line by line) you want to enable verfication for. All the hard work is done by /etc/exim/conf.d/acl/30_exim4-config_check_rcpt.
Let Exim reload it’s configuration and you are good. Nothing more to do. Very convenient, especially for small setups.