I tested Notification with the Test button in the Notification tab and it works.
Then I have a simple script that sends a notification.
When I run it, I receive the notification on the phone, but MT4 hangs and I have to kill it.It happens only on an Azure virtual machine, but it works without problems on my home PC.Sort of same hang issue using SendFTP(). Not to speak about sending mail troubles from there...
We do not provide support for external VPS services.
As you mention the issue is only with your VPS provider so contact them, or switch to a different one.