More about the five types of email addresses

There are five main types of email address and if you want to organize your email sensibly you should know the difference. In this article we’ll talk about the Catch-All, Black Hole and Auto-Responders

Continuing our series on the different types of email addresses.  We’ve already talked about mailboxes and aliases – the most important and powerful type of email address.

All email addresses are formatted the same way @ but what isn’t obvious are the different ways that email sent to an address is handled.

With the basics covered you’ll see how this knowledge can make your email setup much, much better in future Email Essentials issues.


‘Catch all’ addresses can apply when you manage all the email going to a single domain.

Normally messages to an unknown user in a domain are ‘bounced’ with a cryptic message indicating there was no user or mailbox to deliver that message.  You may have seen these messages if someone moves ISP without telling you or there’s a spelling mistake.

With a ‘catch all’ option messages to any unknown or unconfigured username (ie the part to the left of the @ symbol) are redirected to a nominated mailbox instead.

This means the domain owner can give out many different email addresses and not have to configure each one because any unknown email will turn up in his/her mailbox automatically.

‘Catch all’ might seem pointless except if you have the entire domain name at your disposal.  It gives you a lot of options with no ongoing configuration.

The downside of ‘Catch all’ mailboxes is spam. Spammers will often try many different names against a known domain, to see which ones are valid addresses. All those messages will be accepted and delivered to a Catch all mailbox – Ouch.

For that reason, Catch all accounts aren’t used much these days. Instead only email to known mailboxes or aliases are delivered. Anything to unknown users is refused with an ‘unknown user’ or ‘unknown mailbox’ error.


Continuing with from the first part of this column, Fred has defined his mailbox [email protected] as the catch all destination for any unknown messages to the domain.

Fred gives out special email addresses whenever he likes and doesn’t have to configure an alias for each one.  He can make them up anytime.

Fred was asked to give a speech so he nominated a special email address for all messages about the event  [email protected] .   Since that address isn’t defined as an alias or mailbox it gets re-directed according to the ‘catch all’ setting (to the mailbox [email protected])

Black Hole

With ‘Catch all’ there’s the problem of spam to old and now unused addresses.   Once an email addresses gets out there spam can arrive for years later.   At Office Watch we get spam from addresses and domains that we haven’t used for a decade.

You could simply ‘bounce’ the messages with the standard undeliverable response.  Sadly this doesn’t seem to make a lot of difference to spammers who rarely purge their lists when receiving such error messages.  In addition, sending those ‘undeliverable’ responses takes up resources on the mail server and that load can mount up when you’re talking about millions of messages.

The alternative is the black hole – the message is just removed.  No response, no forwarding, nothing and minimal load on the receiving mail server.

You’d only use this in extreme circumstances, you don’t want to send to the black hole messages you want to see.  Only use it when absolutely all messages to that address are unwanted.

Not all mail services have a black hole option and it probably has another name like NUL or Nobody.


Months after Fred’s speech all messages about it have ceased but spam is still arriving to [email protected] so Fred defines an alias for [email protected] (instead of relying on the ‘catch all’ feature) and points the alias to the black hole.


Email auto-responders were common in the twentieth century but the onslaught of spam has made them impractical.  But they still have a place and may get a revival if ever the email system gods manage to reduce spam to a tolerable level.

An auto-responder email address is one that responds automatically, usually a message back to the sender.  Vacation or Out of Office messages are the most common auto-responder. In Exchange Server these messages can be configured to only go once to each person, other messages from the same person are accepted but there’s no response.

Five types of email address, part 1

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