How to Forward Google Apps Email to Secondary Email

One more way to do this in Google Apps is to use a "group". A group is like a mailing list. Here are the steps you can follow:

1. "Manage This Domain" (for domain1.com)
2. "Email Settings"
3. "Users and Groups"
4. "Create a New Group". Fill-in your details. In your case, the group email address will be "guy". Make sure you choose "Team" and "Also allow anyone on the Internet to post messages""
5. "Add new members". Here, you should add "guy@domain2.com"

From now on, whenever anybody sends email to "guy@domain1.com" it will automatically go to guy@domain2.com .

Another way to achieve something similar (but not exactly what you are asking for) is to choose "Manage This Domain" (for domain1.com) --> "Domain Settings" --> "Domain names" --> "Add a domain alias" and then add domain2.com as a domain alias.