After monitoring the situation for five days we confirm that all e-mails are being sent out of MXP successfully again.
The core of the issue has been resolved and we are now monitoring.
We are investigating an issue that is causing some e-mail messages to not reach the recipient. This issue may affect all customers but not all e-mails. We are aware that the issue began at the beginning of September and has slowly grown to affect about one out of twenty e-mails. The issue only impacts instant e-mails, not ones that are scheduled(reporting, etc.) The root cause of the issue is related to enforcing of tighter security controls, therefore we are looking at a way to provide both security and the timely delivery of e-mails.