**Email suppression lists** allow automatic suppression of bad email addresses defined by suppression lists, which will help clean the addresses before sending and thus help you improve deliverability, open & click rates, and inbox placement. Those lists can be found in Project settings (Campaigns -> Channels -> Emails).
## How does it work?
Lists are represented by simple lists of email addresses and domains. In the user interface, the list itself does not differentiate between domain or full email addresses. However, in the background, they are flagged separately to better process them.
Filtering then happens in the email node. _It happens in 3 steps:_
Email address is looked up in the list of email addresses
If not found, the domain is extracted from the address and looked up in the list of domains
If not found, the email proceeds to send
During the send-out phase, email addresses and domains on the suppression lists are excluded from the sending. When any communication suppression happens, it results in creating a campaign event indicating why the email was not sent and will be shown in a specific suppressed state (in scenario evaluation and info).
There are 2 different types of suppression lists and both have slightly different behavior when it comes to populating and suppressing various types of communications:
Global suppression list
Custom suppression list
## Global suppression list
Suppression via GSL is used mainly for spam trap suppression. This list is defined and maintained by our Client Services team & Email delivery specialist (EDS). There is only one version and it’s shared between all clients and instances. Therefore it can’t be customized or changed by the client. Suppression via GSL can’t be overridden by using general consent or via Transactional Email API. This means that there is no way to send an email via Bloomreach Engagement to these email addresses and domains. This list represents a list of domains that are spam traps or other non-existing emails or domains.
## Custom suppression list
The main difference between GSL and CSL is that you can make changes to CSL, and they are separate for every project. By default, it’s empty and can be populated directly in project settings. There is only one limitation - a maximum of 5000 addresses OR domains in total. You can use CSL for email blocklisting. You can add a list of customers who shouldn't get any email by chance. CSL also does not block all emails to be sent - using general consent or Transactional Email API will result in sending an email. This is on purpose based on the nature of such emails which shouldn’t be suppressed in general even if the customer does not wish to receive one.
|Normal campaign email
|Global suppression list
|Custom suppression list
|Email list hygiene
_This table quickly describes how suppressions work with different kinds of email/ suppression combinations._