I am currently running a postfix server with self signed certificates serving one mail domain, mycompany.com, the mail server is mail.mycompany.com and so is the CN of the certificate. Now, I need to add a new domain to it. The new domain name is mycompany.net to the same server.

Since the users already have the root of the old certificate, I'd like to reuse that. However, I'd like to issue a new certificate so users using the SMTP from Outlook/Thunderbird of mail.mycompany.net do not get warnings. If I understand correctly, if I issue a new certificate with CN=mail.mycompany.com and a subjectAltName=DNS:mail.mydomain.net and have postfix serve this, the client will not complain either way about the cn not matching the target host name. Am I correct in this assumption or am I misunderstanding the concept of Subject Alternative Name?

Just to avoid conversation, I do not want to have users on mycompany.net addresses use the mycompany.com server because I might (not a technical issue) have to split up into two different locations, and I want to produce an easily migrateable setup.