Hello,
Currently, my email system with Postfix and Dovecot runs fine. The MX record links to my root A record (let's say domain.com) and I have added to the configuration file:
virtual_mailbox_domains = domain.com

I need to change the A record of that MX record to something like mail.domain.com so that email and regular webiste can point to a different server.

Addinc the A record mail.domain.co and changing the MX record is easy.
But, do I have to change something to my postfix configuration? My limited understanding of emails tells me that the dovecot installation is not concerned with that change.

Thanks
Regards

You won't need to change your configuration. The MX record is used by MTAs to locate servers which will accept mail for a domain. The DNS name for the MX doesn't even have to be on the same domain:

hostname ~ $ dig mx nurfherder.com

; <<>> DiG 9.4.1-P1 <<>> mx nurfherder.com
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 41060
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;nurfherder.com.                        IN      MX

;; ANSWER SECTION:
nurfherder.com.         43200   IN      MX      5 mt-elbert.irgeek.com.

;; Query time: 456 msec
;; SERVER: 10.10.20.2#53(10.10.20.2)
;; WHEN: Thu Jan 24 16:43:45 2008
;; MSG SIZE  rcvd: 65

I change my DNS records and I am done! Thanks a lot for your explanation. Very useful.