Allow for recursive DNS records

[LecrisUT]

Short description:

It is not tested how mailout will react when redirecting to same domain name as the incoming domain. For example we want the rule: *@example.com -> *@example.com, and we have setup the DNS records as:

@ 300 IN MX 10 mail.example.com.
@ 300 IN MX 20 server1.mailout.
@ 300 IN MX 30 server2.mailout.

If the original sender only supports sending through port 25, but port 25 is blocked on mail.example.com, then it will redirect to mailout. The intended result is that mailout will retry the original configuration, but try additional ports.

Usecase:

Simplifying the required DNS records for users, and allow mailout to act as a fail-safe redirecter to the same domain name.

Minimal implementation:

The minimum implementation to get this setting to work would be to detect if the incoming mail is from the same mailout server or not. If there are multiple mailout servers in the DNS record, it can either be that it is a fail-safe mail-server with the same configuration or 3rd party server with different configurations.

• case1: mailout can keep track of the servers that share the same configuration, or add an array of mailout servers belonging to the same cluster. When mailout will try to redirect the mail, it should filter out the DNS records belonging to the cluster
• case2: This is a bit trickier to configure because there can be the same recursive configuration on the 3rd party server as well. Either allow for additional filters to be added depending on the domain-name and have them configured in the mailway front-end, or keep track of the jumps and filter accordingly.