Outbound fails thru Notes - works thru Citrix - only a few users

Here I am again…I have perhaps 5 users who receive Delivery Failure:

SMTP Protocol Returned a Permanent Error 501 5.5.2 invalid Message-ID header

…to just ONE person. Myself & others can send ok but this consistently fails with these few people. If they log onto Notes thru Citrix the message goes fine. What to look for? THX !!!

Subject: Outbound fails thru Notes - works thru Citrix - only a few users

I suspect that the data directory is different outside of Citrix. Verify that the users location documents, security to the drives, etc… are configured the same, inside and outside of Citrix.