Gmail – How to Setup Google Apps for Business Email on iOS

MX records control only the server direction (i.e. messages sent to @example.com will be handled by the server located at 123.123.123.123). It's the local mail server that decides that admin@example.com and mail@example.com correspond to the same inbox. Google uses the term nickname for forwarded addresses.

In order to create a nickname for an existing user/inbox in Google Apps, you need to do the following:

1. Go to your Google Apps admin dashboard
2. Click on Organization and users in the blue navigation menu on top of the page
3. Click on the user, to which you want to assign a nickname
4. In the User information tab (the default one) click on Add a nickname (it's right below Email quota)

You need to create an organisation-wide group from your Google Apps control panel:

Here's how to create an organization-wide group:

1. Create a new group. Learn more
2. On the "Create a new group" page, click Add all users within [organization name] to this group.

The group's member list now includes a special member called All users within organization name.

You can edit the groups access settings or delete it, just like any other group. However, you can't edit the members within the All users member.

Tip: To prevent spam being sent to everyone in your domain, allow only group owners and managers to send messages to a group that has the All users member.

Quoted from Apps Administration Help, here.