Can’t receive meeting requests

My Domain (X) cannot receive meeting invitations from Domain Y.

Email goes back and for the between the two domains all day with no problems as do meeting requests from X to Y…..just not the reverse.

Senders on the Y domain get this error message:

The e-mail system was unable to deliver the message, but did not report a specific reason. Check the address and try again. If it still fails, contact your system administrator.
< (mail server name for Y) #4.0.0 X-Postfix; lost connection with (X mail server name and IP) while sending MAIL FROM>

We are baffled! This problem started abruptly. There were no changes in this domain but we believe that domain Y started using CAC cards just a few days after we got out last received meeting request from domain Y.

Any ideas? we are desperate!