The nullmailer package can be used to setup a forwarding SMTP relay. It does not provide support for local mail, but forwards all messages to one or more remote SMTP servers for processing.

The debops.nullmailer role installs and configures the nullmailer service, optionally with a network access from remote hosts on port 25.

The role can detect other Mail Transport Agents installed on the managed host, and if an MTA from a configurable list is found, the role will automatically disable itself to allow easy switch to different SMTP servers.