My client has dynamic IP on which he hosts Exchange 2010 with POP3 Connector running and gathering emails from his current hosting. Until he gets static IP he wants to send emails out. This will work most of the time but some servers won't accept such email sent by Exchange (from dynamic ip due to multiple reasons) so I would like to make a relay thru external SMTP server which hosts current mailboxes.

Normally SMTP server could be set up to allow relay thru it but this would require static IP to be allowed on that server so it would know which IP is allowed to relay thru it.

Or is there a way to setup relay in Exchange 2010 so it can use dynamic IP and kinda authenticates with user/password itself on the hosted server?