Prova något sånt här:
INSERT INTO list_email($username.$rowname, fname, lname, list_email) VALUES(?,?,?,?,?)
SELECT email_addr FROM list_email AS tmp
WHERE email_addr NOT IN(
SELECT email_addr FROM list_email WHERE email_addr = $post_addr)
WHERE email_addr NOT IN (SELECT email_addr FROM list_no_email
WHERE email_addr = $post_addr)
LIMIT 1;