You may have noticed that there's now a blacklist setting under "User admission" if you have an "open" help desk account. This is used to block users from certain domains. The blacklist is used in conjunction with the whitelist, as demonstrated in the below sample use cases:

Use case 1 - only allow email/signups from a single domain:

whitelist: gmail.com
blacklist: *

The asterisk (*) in the blacklist field is a special value, which basically means "blacklist everything". If the blacklist field is blank, nothing gets blacklisted. The blacklist gets overruled by the whitelist, such that users admitted by the whitelist never get blacklisted, unless there's a modifier in place as demonstrated in the next use case:

Use case 2 - as above, but suspend tickets from a specific user:

whitelist: gmail.com
blacklist: * suspend:random.zendesk.pirate@gmail.com

Ah, you can use the keyword prefix suspend: in your blacklist to override the whitelist preference. There are two prefixes which can be used in your blacklist to manage traffic from specific email addresses, note that the prefixes only are put to use when processing email, and do not affect e.g. sign-up.

Use case 3 - as above, but reject tickets from a specific user:

whitelist: gmail.com
blacklist: * reject:random.zendesk.pirate@gmail.com reject:megaspam.com

The difference between use cases 2 and 3, is that in use case 3 no ticket matched by a reject-rule will show up in the "Suspended tickets" view. Use the reject prefix to completely disregard emails from a specific user or domain.

The blacklist and whitelist settings accept full email addresses and domain names, for example:

Use case 4 - domain/address filtering:

whitelist:
blacklist: suspend:zendesk.com reject:spammer@megaspamcorp.com

This setting allows everyone to send emails to your account, but suspends tickets from anyone from the zendesk.com domain, and completely disregards emails from the specific spammer@megaspamcorp.com address