After contacting the supportdesk i managed to solve the issue, many thanks to the support team of synametrics.

For the people who are having similar issues and see this topic, a quick resume of what solved my problem:

Cause:

* After a look in the XEAMS webconsole -> Tools -> View logs -> SMTPProxyConversation.log, the supportdesk did find the following error "452 4.3.1 Insufficient system resources"

*.* If you have this error this means your Exchange Server exceeded the "Back Pressure" threshold and that inbound mail submission has stopped or in other words the disk where the Exchange database transition logs are stored is (almost) full.

* If you have like me a (homelab) environment with no backup software that is Exchange aware + Full VSS (Volume Shadow Copy Service) support then your transition logs are not being truncated.

*.* In my case i did backup my sever but not with a Backup tool that is Exchange + Full VSS aware the transition logs used 44GB of space.

Fix:

* The easy way expend your disk and wait until free space passes the threshold again. (not recommended)

* Backup your server with a tool that supports Exchange + Full VSS. (such tools cost a lot of money i cant afford for a hobby/learning)

* Do a Full Windows Backup, a feature in Windows Server that can be activated, on the Exchange machine with Full VSS option. The windows backup will do a backup and truncate your logs files.

Note: Its not recommended to manual delete the transition logs because you lose the recoverability of data.