sendmail pop and replaying

Running Redhat 9, sendmail 8.12. I have users in the field who all have a
different ISP. They are having a problem replying to or sending messages to
different users. I am able to send them mail, so they can pop the server,
but when they reply it says that the mailbox is unknown.
If I ask them for their IP and relay just for their IP it works. This will
be a pain to manage because some of these folks are on dialup and their IP
is always changing and I do not want to relay for all of AOL. I can ask some
of them to change their smtp server to their ISP smtp server and this will
work for some but not all (AOL).

Another strange thing is that one user can send to anyone inside the
company, but when they try to send mail to certain people outside the
company they get the unknown mailbox error. If I relay for the person they
are trying to send to it works, so it is a valid email address.

Shouldn't they be able to send/receive mail just fine? Is relaying the
problem? How can I fix this?
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