Question about setting up a DNS record

[Jason Williams]

Good morning everyone.

Well, being is that im a bit rough around the edges with DNS and how to 
configure things correctly, I thought i'd post my question here.

I'll get right to the point.

Our current domain is courtesymtg.com.
Im in the process of rolling out a new mail server for our company. 
However, before I make the switch for the MX line, I wanted to do some 
outside testing to test a variety of things.

So what I wanted to do was setup the following.

I wanted to setup my DNS record to where I can send email to: 
<user>@blowfish.courtesymtg.com

Basically, this will allow me to test all my setups without interfering 
with our current MX record and possibly bringing on downtime. That make sense?

So what I need to do is setup some records correctly, so I can begin 
sending test emails to <user>@blowfish.courtesymtg.com.

Can someone give me some advice here?

I've already setup up an A record for blowfish.courtesymtg.com
Now the trick for me is, figuring out how to setup the MX part of it.

Thanks for everyones help.

Cheers,

Jason