Is there a way to use IMAP instead of POP3 for replies by email?

[pfaffman (Jay Pfaffman)]

@jeanas

I cannot reuse the same domain for hosting the server email notifications are sent from

You can.

@jeanas

it implies I need to purchase a second domain.

You don’t.

I’m not sure what your issue is, but you can use a different subdomain for outgoing mail and the mail receiver. It’s easiest to use the discourse subdomain for the mail receiver, but it’s not really required.