Emails has exceeded the max defers and failures per hour

I have a system that is running for practicly 4 years now. But since yesterday my provider is blocking the emails send. Wich is a real pain in the but. I don't know if something has changed or if their is a update I am unaware of that might fix this issue?

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

  [email protected]
    Domain xxx.be has exceeded the max defers and failures per hour (50/50 (100%)) allowed. Message discarded.

I believe the issue lies with the provider but they claim it might be me who is sending emails in a loop. As this issue occured only a few days ago and I have not uploaded anything I strongly doubt it is me who is at fault. But I ask here just to verify I am not overlooking something.