In the Card Not Present section of BBMS the email address is a required field.  However, this is not a required field when processing a credit card in the Card Present section.   If processing a Card Not Present transaction you can enter any address in the Email field.  The email address does not get checked and no email is sent to the address.

If you would like to send an automatic email acknowledgement for transactions processed in the Virtual terminal, enable "Send virtual terminal acknowledgement" in How to print Receipts or Donor Acknowledgement Letters in Blackbaud Merchant Services