Mutual MX with friends

Hi out there,

I got a stupid question after setting up MX and similar stuff on my DNS:
Suppose the following:
Two people run a Nethserver instance. Both persons own an DNS (name-a.tld and name-b.tld) pointing correctly to their IP. Both are independent from each other and each creates a virtual mail domain (name-a.tld and name-b.tld) within the nethservers email module DNS. In short words: Each person A and B does have a valid E-Mail server vor his/her organisation (like mine :-))

Now person A creates an aditional email domain with name while person B does the same vice versa. During setup, each person sets up the other persons as the relay server:

Example for Persona A:

Now both set an MX Record pointing to the other IP / domain name with a priority value higher the own server.
MX Record of person A would point to mx.person-b.tld or in case of an A * Record just to person-b.tld. Would this be equivalent to an MX service?

THX for comments

I guess this would only be viable for the clients that use the NethServer instance as their DNS server. For the rest of the world this is not going to work since they don’t have the NethServer as their DNS server.

I am not shure if that senario would work: Internal mail would not be delivered at all (because the server filed), while external mail would be redirected to the MX due to the external DNS Record.