Marketing Cloud SAP has been configured and domain has all the usual signatures in place. However, does the sending email address need to actually exist as a mailbox?
No. The important part of SAP and from address management with regards to deliverability is DKIM / Domain Key Identified Mail. That's why sticking to the authenticated subdomain is crucial, but everything before the "@" in the from address is up for grabs.
- once you have an SAP, all emails on the relevant subdomain are verified automatically, as you found.
It is generally recommended to stay consistent in your setup throughout the IP warming (=use the same from address throughout), but there is no check whether there is an actual inbox behind the from address.
There is no handshake between sender and recipient in email processing. A recipient of a mail has to rely on the mail header, and otherwise would not be able to check if the sender address existed. For that, the recipient would have to "try and reply". This would have to be done for each email, which would result in a LOT of replies. This isn't happening.
The actual, technical from-address anyway is not the same as the from-address you specify - in an email header, you'll find this sort of thing, specifically WITH an SAP in place, which removes the "exacttarget" references here: smtp.mailfrom=bounce-153_HTMLfirstname.lastname@example.org
having a real mailbox behind the sender is irrelevant from a business perspective as well. That's what reply mail management is for.
and the weakest point, but probably also useful: I have been part of several IP warmings with non-existing from addresses like "email@example.com" and never had issues.
hope this helps!