Question : Lotus Notes users (version 6.5) get authentication dialog getting mail from Exchange 2003 server: why?

I have a client that is getting an authentication required dialog box when he gets mail from me.  I have my own Exchange 2003 server.  I am not sure where to start trouble-shooting this issue as I know little about Notes.  I have don't network administration with Exchange for years, but no one has complained of this before.

Answer : Lotus Notes users (version 6.5) get authentication dialog getting mail from Exchange 2003 server: why?

What kind of messages are you sending? Is is a regular mail or some sort of forms? Do you sign the messages using S/Mime?