To quote the thread where the mod was announced:
Subject New Mod -- Ban Email Domains
Posted by JPDeni (Veteran)
Posted on 20-Jun-00 03:08 AM
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I just finished a new mod which will allow you to keep a list of email domains that you wish to ban from your site. It requires the installation of the secure password lookup mod first.
This mod will allow you to prohibit users who have certain domains in their email addresses. The most common use of this is to ban free email domains, requiring users to give their true email address. This mod will also allow you to edit the list of domains within the DBMan script, so you don't have to download the file, edit it and then upload it again.
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So you do need to first install the secure password lookup mod before banning email addresses.
Hope this helps
Unoffical DBMan FAQ
http://webmagic.hypermart.net/dbman/