[TODO]: Refactor Relay host support

Description

Creating an actual issue for tracking this.

I recently responded to a support question referencing the feature guidance in our docs: https://github.com/orgs/docker-mailserver/discussions/3605#discussioncomment-7418198

I've included a technical overview of the file in it's current state, while the file itself has plenty of information in comments and git blame should provide more from the original PR of mine that introduced them.

It would still be best to delay until the test suite can adopt compose.yaml + DNS approach for a proper relay test between DMS instances.

UPDATE:

• New reference comment for relay.sh context
• Jan 2024 PR providing another refactor iteration, with some relevant context links in the description (also see the revised relay.sh comments).

Additional issues discussing future improvements to consider supporting, such as recipient transport switching via transport_maps (3860 briefly mentions the sender equivalent variant as well):

• https://github.com/docker-mailserver/docker-mailserver/issues/1470#issuecomment-619367183 (EDIT: Nothing useful here that isn't already covered by the others AFAIK)
• https://github.com/docker-mailserver/docker-mailserver/issues/2601#issuecomment-1135520225
• https://github.com/docker-mailserver/docker-mailserver/issues/3860#issuecomment-1920026734 (solid resource and discussion on the topic/details)

Related relay functionality issue to tackle after that: https://github.com/docker-mailserver/docker-mailserver/issues/3532

Start with relay.sh, its comments, git blame, and the linked reference discussions and PR context. Review how the test suite could use compose.yaml and DNS for a relay test between DMS instances. Done means the relay support is refactored and covered by the intended inter-instance relay test, but the issue leaves the exact scope open.