I've had a couple people ask me this question the past week :
- My users are not getting their email but it's only a problem for a few senders. Help!
- Are all of the senders sending from the same domain name or email provider?
- If they are from different ISP's what are their domains / outgoing mail server IP addresses?
- What do your mail logs say?
- If there is nothing in your logs what do their outgoing mail server logs say?