I have a DNS question!

In a Microsoft world,MS recommends that an internal DNS domain name take
the name "mycompany.LOCAL" to prevent any external DNS issues.

I recently setup my SBS 6.5 server and set the internal DNS using
mycompany.COM (not .LOCAL as MS Suggests).

Now I have an issue whereby my mx record sitting on an EXTERNAL DNS
server, points to "MAIL".mycompany.com, but my SBS server has an internal
host-name of "XYZ".mycompany.com. My DNS guru tells me it was a big
mistake to call my internal network the same as my external domain name.

Who is right and does anyone have any recommendations? (Could I or should
I attempt to change the internal DNS name of my SBS system, or just change
the mx record to point from MAIL to XYZ.mycompany.com.

I looked on the knowledgebase on Novell's site, but didn't see any TIDs
relating to best practices DNS setup.