certain email addresses do not receive esign documents

I have certain email addresses that will not receive documents from esign services.  The email addresses in question are within my company's domain.  I have checked the typing of the email address several times and it is correct.  I can send other emails to those same email addresses with no problem.  The only issue is when routing to them via esgin, the documents never arrive - not to their in-box, not to their spam folder, not to their junk folder, etc..  I hope someone is able to assist.