I sent a message to the listserv group Friday (a few weeks ago). I
received a message from the software saying it had successfully been
delivered; however, several of those people did NOT receive the message,
including my own personal email account. 

Last week:
Despite a message telling me that the message had been successfully
delivered on Friday, I sent individual messages to each person on the
listserv and they DID NOT GET THE MESSAGE. Also, at least ten of them
told me they have NEVER received a message, at all!

I have read manuals on the site but cannot figure out as to how I should
approach this problems. I would appreciate any help and tips possible.

Thank you.