This page assumes you already know how to make an editable copy of your zone file.
If you don't, you might want to see this step-by-step tutorial first.
MX records are what are used to point your domain name to the servers where your mail is handled.
Gandi, as well as most web hosts, provide mail hosting as part of their package. To link your domain to your mail hosting, just change the MX records in your domain's zone file to be those provided by your host (Gandi adds ours automatically when you register a domain with us, as part of our default zone file).
MX records take the following form, and there are usually more than one in a zone. Here are Gandi's as an example:
@ 10800 IN MX 10 spool.mail.gandi.net. @ 10800 IN MX 50 fb.mail.gandi.net.
If you are using Google Apps for your domain, you can see the Google MX records here.