Catch-All Addresses:

To forward all email to non existing accounts on your server to a specific email.
For example, if your site is www.someone.com and you have one email setup on it which is for instance webmaster@someone.com, if someone sends an email to admin@someone.com  it will be normally returned to sender because the address doesn't exists, but this isn't the case here, catch all addresses enables you to receive all email to your site even if the address is wrong providing that @someone.com is correct.
 

Auto-Responders:

When someone emails you the Auto-responder will reply to the sender with a pre-defined mesaage you will setup in your control panel.

Mailing Lists:

To create lists of emails for you to send to from a specific email, this is a useful feature for newsletters.

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