This:
$ host mailgate.compuserve.com mailgate.compuserve.com is a nickname for mx3.compuserve.com mx3.compuserve.com has address 149.174.206.135 mx3.compuserve.com has address 149.174.177.136 mx3.compuserve.com has address 149.174.217.133 mx3.compuserve.com has address 149.174.217.137 mx3.compuserve.com has address 149.174.217.136 mx3.compuserve.com has address 149.174.217.135 mx3.compuserve.com has address 149.174.177.134 mx3.compuserve.com has address 149.174.177.133 mx3.compuserve.com has address 149.174.206.137 mx3.compuserve.com has address 149.174.206.136
...is NOT a variance from RFC1123 or any other specification. It is completely appropriate for a mail domain to be a CNAME pointing at a handful of A's. It's not ok to _advertise_ one of these, as for example in an exported "From:" header, but there's no problem just from being willing to accept mail sent to such a domain or to set up DNS in this way.
Make the DontExpandCnames option true in your /etc/sendmail.cf, and when customers start bitching that they can't send mail to CompuServe addresses, tell them that it is CIS's fault, and give them the CIS customer service number :)
Please don't do that. CIS is committing some real sins against the RFCs elsewhere, and since this isn't one of them it would muddy the waters if we contrive to get them yelled at for it.