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Google Apps email is great!

As a web developer, you will likely at some point have a script on mydomain.com, that sends email to myuser@mydomain.com. If you have Google Apps Email set up for myuser@mydomain.com mydomain.com's MX records, you won't receive the mail because your mail transfer agent will just put your mail in myuser's mailbox on the server, where myuser will never see it.

If you're using postfix, you can go into /etc/postfix/mail.cf (or similar) and remove mydomain.com from the configuration entry mydestination. Then all mail will be delivered as usual.