I'm running Exchange 2003 sp2 in a larger e-mail organization with multiple Exchange Servers.When a user sends an e-mail outside the company and specifies both an internal recipient and an external recipient in the "To" field, some external e-mail servers refuse the connection with an error similar to the one below:There was a SMTP communication problem with the recipient's email server.Please contact your system administrator.<cinemail1.na.FKILogistex.Local #5.5.0 smtp;550 Authentication required.Sometimes that error includes comments about relay attempts but I believe it's being caused by the same phenomenon. If they send the e-mail with only the external recipient specified and not the internal recipient, the e-mail is delivered fine.This only happens when sending to some external servers not all.We have appropriate rDNS records setup.E-mail is sent out using public DNS lookups.No smarthost or mail proxy.It's almost as though the receiving, external e-mail server thinks we're trying to relay the other addresses it doesn't consider local.Any help is appreciated.

KevinStaton wrote: There was a SMTP communication problem with the recipient's email server.Please contact your system administrator.<cinemail1.na.FKILogistex.Local #5.5.0 smtp;550 Authentication required. Above error explains that recepient having Micosoft Exchange server 5.5 . Reason for this error is 5.5 dont have reverse DNS look up feature.