Manage your domain
Once your domain DNS records are set up, you can then create aliases with it.
On New Custom Alias page, you can now see your domain listed.
If you go to your domain detail page and click on "Info", you can set different options. The most useful one is probably "Catch All" that would allow you to just use email@example.com next time you need an alias: it'll be automatically created the first time it receives an email.
Unlike a normal alias, a deleted custom domain alias can be restored. You can see the list of all deleted alias by clicking on the "Deleted Alias" menu on the right.
For a fine-grained control over the auto created aliases, feel free to click on "Auto Create". Although it uses regular expressions, you don't need to be a regular expression guru to use it.