multiple replies to [email protected]

In recent days, I came over strange behavior:

when I send a test message to [email protected], I am getting not only one reply, but 4 up to 10 or so in irregular time steps during some following 5 hrs or so.

Tested with more devices.

While the permanent reception of messages can be annoying, I hope at least it is not counted as messages on the plan.

Someone experiencing similar?