Good morning to you too Sir,
I'm not great at explaining certain things, although I understand how they work so i'll do best to explain here.
As I see it, the conflicts would be happening as you would've called your server "www". Historically, www was a DNS prefix referring to a service. So, when you typed it in, it would take you to the www service on that server, much the same as ftp would direct you to the ftp server and pop would direct you to the pop3 server... and so on.
On your server you've created the domain fiefdom.com as you state, and you're only using that for mail so you've called the server (assigned the hostname) www. You'll still have virtualhost files for Apache and a www A record. My point here is you have a service running on your server for Apache and it would be accessed via the domain itself, or the www prefix. This is where some confusion for your server or errors could happen as when accessing a server by it's hostname, it's unusual to be accessed under the guise of another service (if that makes sense)
Regarding the TLS cert for the server, I can't stop you calling the server "www" but it's your choice. Yes that cert you have can be used to secure the hostname of www, and the services attached to the hostname, but when you try and access the mailbox via mail.fiefdom.com there won't be a certificate to cover that particular service FQDN as you only have a single domain, not a wildcard or SAN.
In short, a server should be identified by and called something totally different from any domains you host on the server, and other services. If you must use one of your existing domains, give the hostname a unique identifier and FQDN A record.
I'm not great at explaining certain things, although I understand how they work so i'll do best to explain here.
As I see it, the conflicts would be happening as you would've called your server "www". Historically, www was a DNS prefix referring to a service. So, when you typed it in, it would take you to the www service on that server, much the same as ftp would direct you to the ftp server and pop would direct you to the pop3 server... and so on.
On your server you've created the domain fiefdom.com as you state, and you're only using that for mail so you've called the server (assigned the hostname) www. You'll still have virtualhost files for Apache and a www A record. My point here is you have a service running on your server for Apache and it would be accessed via the domain itself, or the www prefix. This is where some confusion for your server or errors could happen as when accessing a server by it's hostname, it's unusual to be accessed under the guise of another service (if that makes sense)
Regarding the TLS cert for the server, I can't stop you calling the server "www" but it's your choice. Yes that cert you have can be used to secure the hostname of www, and the services attached to the hostname, but when you try and access the mailbox via mail.fiefdom.com there won't be a certificate to cover that particular service FQDN as you only have a single domain, not a wildcard or SAN.
In short, a server should be identified by and called something totally different from any domains you host on the server, and other services. If you must use one of your existing domains, give the hostname a unique identifier and FQDN A record.
