Sendmail doesn't work for certain domains?

Sendmail doesn't work for certain domains?

I have a Sendmail statement at the end of an onClick action of a button on a stateless form.  I have found when testing the system that when the email address is a gmail account, the email arrives as expected.  But when I send emails to addresses in our own domain (eg. bob@mydomain.com) they disappear - no errors, no spam, not caught in the email filter.  They just vanish.  Any ideas about potential causes?  Settings on our exchange filter that would block these?

Thanks,

Peter