We are running a Exchange 2010 and 2013 coexistence.  Mail Flow is fine on the 2013 side, but for mailboxes still on 2010, some are getting mail delivery failures only for certain external addresses.  One of the deliveries says the Sending IP does not have a valid rDNS.  The IP Address it sent back is the main Public IP and not the Public IP assigned to our mail servers.  Not understanding why the 2013 mailbox users can send to these external addresses no problem when 2010 mailbox users are being routed through 2013.  Thanks for any help.

You don't have any SendConnector with a Exchange 2010 listed as a source server?

Hi,

According to your post, I understand that account in Exchange 2010 deliver message failed for certain external addresses, however the user in Exchange 2013 works fine.
If I misunderstand your concern, please do not hesitate to let me know.

Whether all user in Exchange 2010 or some special account experience this question?
If this issue only occur on some specific external address, please contact its administrator to double confirm the configuration of anti-spam filter.

Besides, I recommend run Message tracking log and Protocol log to check the deliver process. More details about Analyzing the protocol logs and Message tracking logs in Exchange 2013:
http://social.technet.microsoft.com/wiki/contents/articles/23182.analyzing-the-protocol-logs-and-message-tracking-logs-in-exchange-2013.aspx

Thanks