Are you sending mails thru an email server or just an SMTP service? When emails are sent from applications (like your CMS) usual choice for delivering the emails are
- Email Server on the network (Exchange, others)
- An SMTP service (IIS SMTP, others)
Here is the catch. If the application (your CMS) is in the same network as an email server, then this is the preferred choice, since the email server is the desiginated post-office for that domain. If on the other hand, the application has no access to relay emails thru a mail server and an SMTP service is used then note the following
If the SMTP will send directly to external users then you need to make sure the IP(public) is cleared from any blacklists, make sure you have correct PTR records are setup for reverse DNS by other mail servers, and for heavens sake turn off relay.
In your case, if the person on HOST A is on the same network as the CMS then he/she will get the mail. If HOST B is on a separate network, perhaps HOST B mails server is doing a reverse lookup and is unable to determine if the SMTP is a legit mail server. You also need to have proper MX setup.
If both users are on the same network, then it could be that the sender (from address e.g. firstname.lastname@example.org) is not allowed to send to that user. Can't you ask your email admin to put a trace on the email? (that's if you are sending from a mail server.