1. It depends but the max is around 20 per hour (which is not that often).

2. I think it was caused by one message and every message after that also stayed in the inbox. That one message contained PDF attatchements (maybe it’s a clue for the problem).

3. The attachments combined were 4,6MB

4. I’ve set it to 5 minutes now just to be sure, but it has worked for months with a 1 minute cronjob