If you would like to Exclude an entire Domain for your Mailing, you can simply add a Wildcard to your Exclusion List – for example if you would like to Exclude all recipients with the Domain yahoo.com.

To do this, please go to Groups, Exclusion List then here add @yahoo.com and choose to exclude from one Group or All Groups. This is the Wildcard. Now when you send to your Group, all @yahoo.com email addresses will be excluded from this mailing. 

You can do this for any domains you want to exclude from the mailing.