Bulk Mail Contacts

This usually happens if internal email address field is kept empty. Please make sure that you have specified the value for internal email address for contact. Please note that value of domain for internal email address must match your accepted domain in Exchange.
 
For example, your Exchange Accepted Domain ( Mail Domain) is domain.com then in this case, in csv file you must enter the same domain name for internal email address like someone@domain.com . I hope after specifying correct internal email address in csv file against your Mail Contacts, the issue will be sorted out.