In general there is no direct link (in the database) between ONE web domain and email addresses!
If you want to access a list of emails that MIGHT(!) be connected to a given WEB domain you could try doing a join like this.
SELECT email FROM mail_user INNER JOIN web_domain ON (SUBSTR(email, INSTR(email, '@')+1) = domain) WHERE web_domain.domain_id = 12345
It's not guaranteed to work because of the things I stated above.