I am about to transfer webhosting for a domain to Dreamhost, but the client already has their own mail server which won't be changing, so I want to make sure I understand this correectly so I don't screw up email for the 20 or so people who could be affected.
The way I understand it, I would set up a custom A record for "mail.domain.com" that points to the existing mail server's ip address, and then a custom MX record of "10 mail.domain.com" through the dreamhost control panel.
Is this correct? And if I do this before I swich the DNS servers to Dreamhost (DH is not the registrar) will the transition be seamless?
Also, if I later transfer my domain registration to DH, would I have to change anything?
I'm sorry if this is a basic question, but I've never done it before and I don't want to risk losing email for 20+ people.