You can see them in the Bounced email report at yoursite/admin/email/bounced
Or you can use data explorer for finding all users of a domain, then export a csv file. maybe something like:
-- [params]
-- text :domain = example.com
SELECT u.id AS user_id, u.username, ue.email
FROM users u
JOIN user_emails ue ON u.id = ue.user_id
WHERE ue.email LIKE '%' || :domain
AND ue.primary = true
ORDER BY u.username
Or you could make a group and automatically add them to the group
