If you find out that you are not receiving e-mail messages that had been sent properly, there could be a few different reasons:
To try and reduce spam, some ISPs block e-mail being sent by specific servers. The sender should be getting a delivery failure notice indicating that the message was blocked and why, such as:
<username@doman.net>:Connected to nnn.nnn.nnn.nnn but sender was rejected.Remote host said: 550 nnn.nnn.nnn.nnn blocked by ldap:ou=rblmx,dc=domain,dc=net -> BL004 Blocked for spam. Please see ...
How to get the IP address unblocked depends on the ISP. Examine the delivery failure notices (have one of the message senders forward one if necessary) to see if there is a link that provides further information.
Comcast: