Is it possible to make Postfix accept $myhostname = domain.tld (not host.domain.tld)?

Two interrelated questions:

1. I'm trying to understand if it's not recommended to have a server hostname set to just domain.com instead of host.domain.com (system-wide). One consequence of that for me is that Postfix stops working properly, complaining of user not being found when an email is being received: status=bounced (unknown user: "mailuser") where "mailuser" is a part of [email protected]. I'm not sure of the exact cause of that particular problem (could be that it's tied to Dovecot's user database), but the question is about whether setting the hostname like that is not recommended in general and why.

2. My problem is that I would like to enable SSL certificates for the server on the receiving end and I only have the certificate issued for domain.com and www.domain.com and not host.domain.com. I'm trying to work around that by making postfix reply to HELO with "domain.com" and subsequently present a valid certificate for "domain.com", but haven't found a way to do so without breaking mail delivery. Right now it presents a certificate for "domain.com" while announcing itself as "host.domain.com".