The <DiscQuery> element is the SQL query issued to find all unique domains that are serviced on the verification server. This list of commands must include any domain (such as an alias) for which the server can ever successfully verify an email address. The query should return one row per domain, each row containing a single column containing the text of the domain name.
The following code snippet shows how to retrieve the unique list of domains from a table that has emails already separated into a user column and a domain column:
SELECT DISTINCT domain FROM mail_table
The next code snippet shows how to retrieve the unique list of domains from a table which has a column of full email addresses:
SELECT DISTINCT mid(email_addr, instr(email_addr,'@')+1)