Is it bad practice to declare MX from differing networks?

Solution 1:

They are mostly wrong.

It is not a bad practice to have more than one MX, and it's equally not a bad practice to have one or more of them with a hostname in another domain. In fact, it used to be quite common that people would set up their own mailserver in their own domain as their primary MX, and then have their ISP's mailserver as secondary MX.

The one tiny part that might conceivably be relevant is that if the MX in the other domain doesn't resolve properly, e.g. if domain example.net is having DNS issues, that would be an issue. But that's why you have more than one MX - if one fails, the others will still work.

You should respond to the provider and point them at RFC 5321, section 5.1. It's a bit too long to quote, but the gist of it is that if there's more than one MX, the sender must try at least the first two, and there's no restriction on having them in separate domains.

Solution 2:

No, this is BS. Having this option is one of the main reasons why you can specify multiple MXs with different priorities in the first place.

There must be another problem.