Email sent to external recipients between March 27 and April 1 had improper Reply to addresses.
Starting Friday, March 27, some Exchange users began to encounter a problem where emails sent to external recipients contained malformed Reply-to addresses. When recipients reply to these messages, the reply fails, triggering “undeliverable” notifications. Messages sent to internal recipients are not affected by this issue.
As of midnight, Wednesday, April 1, normal operation has been restored. Note, however, that replies to problematic mails sent after this date and time will still trigger "undeliverable" notices.
This problem came about after a routine software update, and affected only those users with accounts hidden from the Global Address List (GAL). Students who have not published their email address in the Student Directory will be affected.