Mailman not accepting messages

In preparation of the migration to Microsoft Exchange Online Protection (MS EOP), we added another mail exchange server (mx5) to be addressed directly by MS EOP. However, having had mx4 in production for some time, we forgot to test mx5 in conjunction with our mailman server (zaaivm). This could result in a bounced mail ’not accepting messages’ for FNWI users (employees and students) using @ru.nl addresses.

Meanwhile, mx5 has been made known to zaaivm thus resolving this issue.