Proxy pipeline step

[foxcpp]

As an SMTP pipeline step:

proxy address0 address1 address2

Addresses use the same syntax as config definitions for endpoint modules. Remotes are tried in specified order, no more remotes will be tried after a successful transaction.

[foxcpp]

I guess it could be better to add forwarding functionality to the remote module. It could benefit from sharing code for MTA-STS policies, for example (#81).

Defaults, send message depending on the recipient domain and target MTA discovered using MX records.

deliver remote

Skip target MTA discovery and just hand all messages to the first working MTA in the list.

deliver remote {
  forward_to ADDR1 ADDR2 ADDR3
}

Basically the same, but apply MX-based discovery.

deliver remote {
  foward_to_mx DOMAIN1
}

Additional directives, extending the remote module in general:

• auth USER PASS
  Authenticate using SASL PLAIN to the remote server.

• auth forward
  Pass local credentials to the remote MTA, causes an error if the client is not authenticated.