Let's say you have an SAP and the sendout subdomain: yourSubdomain.domain.com
A customer receives an email and hits “reply”.
1) Typically, in the "to"-Field of the reply they will see what you define as "Email Display Name". You can enter anything here as long as you're ok with your customer seeing it. It doesn't have to be formatted like an email. No problem here.
2) Reply subdomain can be the same subdomain used for sending, which makes things easier. As it is in your current setup and actually a lot of implementations. This subdomain is where the whole reply processing logic of "filtering out autoresponders" etc. sits. If you simply change this to any domain, RMM will not work. Just think of it like this: How would a random domain know how to process a reply (e.g. filter out autoresponses) for you correctly? It wouldn't.
The reply subdomain needs to understand the following -
This is what some ESPs also surface to the customer, a bit of a "technical" looking, dynamic link that is part of the email header of a sendout from SFMC, which looks like this:
The way the link looks is necessary to tell the reply subdomain how to process the reply. If you want RMM functionality, you thus have no more influence on how this link looks after defining the subdomain as part of SAP.
Now there also is DMARC "Forward Reply Header Rewrite" - but if this is the source of the change request, you should contact Salesforce for the next steps, this is not something you just change in RMM settings and it is a whole new topic. Resources:
3) The third required field is Email reply address.
You can enter ANY address, also on the reply subdomain here, it usually doesn't matter. Let's say "service@yourSubdomain.domain.com" - the only important part is, it has to have the syntax format of an emailaddress so you can enter it in the field. Why "does it usually not matter"? If you use filtering /redirection of replies (like most systems are set up), no customer will ever see this and nothing will go here. The customer will see the name from 1), the link from 2) and the reply will reach the inbox from point 4).
And in this case you don't have to have a real or monitored inbox here.
ONLY if you do NOT want to use any filtering /redirects as part of RMM, but direct forward, the content of this field will have any effect, meaning it will be shown to the customer instead of the link from 2), and it will receive unfiltered replies.
But without the filters, you would get tons of autoresponses etc. to this address. I have never seen this implemented at scale, as this inbox would receive thousands and thousands of meaningless responses. A big reason for RMM is to avoid this.
4) The fourth part is the "Routing Address for Remaining Replies"
This is where the filtered responses are actually forwarded after going through the RMM processing implemented on the reply subdomain.
As it is a forwarding logic here, your customer will not see this address anywhere when they reply. This is where the replies go after RMM processing. THIS should be a real, monitored Inbox. It should not be on the sendout subdomain.
Hope this gives some context. It's really not a well explained feature, and I hope this helped a bit.