The Catch-All Alias is a special kind of forwarding address. If you turn it on, it accepts mail sent to any address that you haven't specifically created.
For example, imagine someone sends an e-mail message to unknown54@example.com. If you have turned on the Catch-All Alias, the message will be forwarded to the address you've chosen. But if you don't have the Catch-All Alias turned on, the message will be rejected with an "unknown user" error.
The Catch-All Alias is a useful way to receive mail for many addresses without needing to add each address individually. It's also useful if you want to receive any misaddressed mail: for example, using the Catch-All Alias would make sure that you receive a message accidentally sent to inof@example.com instead of info@example.com.
Although the Catch-All Alias can be useful, it has a (possibly major) drawback: If you use it, you'll probably receive much more spam, because some spammers send mail to random addresses such as sales@example.com, jobs@example.com, list@example.com, president@example.com, and so on.
You'll have to decide whether the extra spam is worth the convenience. We don't usually recommend it for accounts that receive more than a few e-mail messages a day, but it's up to you, of course.
If you want to turn the Catch-All Alias off to stop extra spam, just do this:
Due to virus and spam outbreaks, our mail systems won't accept most "bounce" messages sent to Catch-All Alias addresses (unless the spam filter for the Catch-All Alias is completely turned off, but don't do that). This makes sure you won't be overwhelmed by erroneous "bounces" if a spammer or virus sends other people messages using a random address at your domain name as a forged "From" address. However, you might still receive erroneous "I'm out of the office" autoreplies and similar messages; the only way to stop those is to disable the Catch-All Alias completely.
Rejecting "bounces" sent to catch-all addresses should not affect normal mail, because you'll normally send outgoing mail from a real mailbox you've created. Any "bounces" will be returned to that mailbox, not to a catch-all address.
In the unlikely event that you are sending mail from addresses that aren't real mailboxes, our system will try to track the outgoing messages and remember the address you used so it can accept future "bounces". For greater reliability, you should also add a normal forwarding address to handle "bounces" that arrive for the address you're using in your outgoing mail. That will ensure that any "bounces" aren't rejected by the Catch-All Alias.