Please note: For information on how to exclude particular contacts from your recipient list, see another article: How to create a suppression list.

There is a way to exclude a whole domain from your recipient list that will require just a little work on your end.
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Creating a segment that stores all the contacts from a particular domain is a good way to manually exclude such domain from your future campaigns. Keep in mind that this method will work only if these contacts are already present on your account's contact lists.

A segment set up like this will automatically pick up all newly added contacts that contain "domain.com" for your future review.

When you successfully gather contacts from one domain in such segment, you then can freely change this whole segment's status to inactive. Our system prevents sending to inactive contacts. You can learn more about contact statuses here.
First, select all the contacts using the top-most checkbox in the segment and and then change their status in the resulting pop-up to "inactive".

The step of status changing can be repeated multiple times every time a new contact with the specified domain appears on the segment so that you can ensure you won't send to it.