Note that this is not recommended. It breaks the ability of the mail server to reject mail early in the delivery process, which means your system will work dramatically harder because full spam messages and viruses will have to go through the full processing chain. It is far better to create every alias you need, as you need them.
- Choose the domain name of your server from the pull-down menu
- Click "Edit Mail Aliases"
- Click "Add an alias to this domain"
- Set "Name" to "All mailboxes"
- Fill in the delivery settings as usual (a local mailbox, usually).
- Click "Bounce mail?" if you want to use this catch-all email account to bounce inbound emails with invalid email addresses.
To "turn off" the automatic email bouncing, simply delete this catch-all email alias.
That can occur if your hostname gets out of sync with what's setup in your server's config files.
To check for this problem, log into your server as root over SSH, and run the command "hostname". Then, verify that the resulting hostname exists next to your external facing IP address in /etc/hosts, as well as on the "mydestination" line of /etc/postfix/main.cf. After making those changes, restart Postfix.