> Now the ISP has been nice enough to point the MX records for these
domains
> to the server at IP A.B.C.D

There's your problem. IP addresses need to be numeric, not alphabetic.
The server address should look something like 242.245.236.64 not
CAT.DOG.HORSE.COW, unless you're using IPv6, in which case your IP
address appears to be invalid. 

> Now a rev DNS at say dnstuff.com on @myco.com reveals indeed the PTR
is
> there for myco.com

Agreed:
myco.com        MX preference = 5, mail exchanger = avmx.atgi.net
myco.com        MX preference = 10, mail exchanger = mail.myco.com
26.194.174.216.in-addr.arpa     name = avmx.atgi.net
26.194.174.216.in-addr.arpa     name = avmx.atgi.net

> BUT a rev DNS on yourco.com revels the PTR to the ISP

Which one is the ISP?
yourco.com      MX preference = 10, mail exchanger =
tinysmtp2.timeinc.com
yourco.com      MX preference = 20, mail exchanger = nycmx1.timeinc.com
yourco.com      MX preference = 20, mail exchanger = nycmx2.timeinc.com
yourco.com      MX preference = 30, mail exchanger =
tmpmailws01.timeinc.com
yourco.com      MX preference = 40, mail exchanger =
lonmailws01.timeinc.com
yourco.com      MX preference = 10, mail exchanger =
tinysmtp1.timeinc.com

242.245.236.64.in-addr.arpa     name = tinysmtp2.timeinc.com

> One User has yourco.com as their reply default so.... [EMAIL PROTECTED]
> 
>  now this suser Jabne sends an e-mail to a friend at AOL..now AOL does
a
> rev
> lookup for spam right?...

Maybe?

> So the e-mail bounces back with the stuff from AOL indictae you may
not
> relay thru..bad

Dude AOL sucks, that's a horrible error message. I'm putting them on my
RBL until they fix that. 

> Am I correct is assuming that the reason it is being bounced back is
> because
> AOL can not rev look up YOURCO.com because the ISP has NOT put in the
PTR
> for yourco.com? To point back to the server at A.B.C.D

Some domains do a PTR lookup for various things. The Mx record for
yourco.com need not be a yourco.com domain. As the results above
indicate, at least one of the Mx records for yourco.com has a valid PTR,
forgive me for not checking them all, but I'm pressed for time.
Regardless it is typically the sending IP which needs a valid PTR and
the address provided is invalid, so further troubleshooting is not
currently possible. 



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