Ubuntu: sudo apt-get install postfix

• During postfix install process, the configure text dialog will display five options: "No Configurartion", "Internet Site", "Internet with Smarthost", "Satellite Systems", "Local Only".
• Select "Local Only".
• For the domain name, use the default suggested and finish the install.

Enabling this, you can use any email address ending with "@localhost".

Example: here, my unique account is rael@localhost. But while testing systems, I can use any address like joe@localhost, foo@localhost, etc, because all will be redirected to rael@localhost

• If not exists, create file /etc/postfix/virtual: sudo nano /etc/postfix/virtual
• Add the following content, replacing with your user account: @localhost <your-user>
• Save and close the file.
• Configure postifx to read this file:
  • Open /etc/postfix/main.cf: sudo nano /etc/postfix/main.cf
  • And check if this line is enabled, or add it if not exists: virtual_alias_maps = hash:/etc/postfix/virtual
• Activate it: sudo postmap /etc/postfix/virtual
• Reload postfix: sudo service postfix reload

Ubuntu: sudo apt-get install thunderbird

• Skip the first welcome screen (click in the button to use existing accounts);
• Click in the Settings button at top right (similar to Chrome settings) then click on Preferences > Account Settings
• Under Account Actions choose "Add Other Account"
• Select "Unix Mailspool (Movemail)"
• Your account will be <your-user>@localhost (of course, replace <your-user> with your user account). Don't use <your-user>@(none), use <your-user>@localhost
• Ingoing and Outgoing server will be: localhost
• Restart (close and reopen) Thunderbird.

• Send new email to <your-user>@localhost, replacing <your-user> with your user account
• Click on "Get Mail"
• Test catch-all: send new email to averagejoe@localhost
• Click on "Get Mail"