when I eliminate users from the ACP, the following error message appears:



and it will only let me eliminate up to 4 users at a time

which could be the problem?

Thanks

Apparently the problem is: "Ran into problems sending Mail. Response: 452 message sending quota exceeded, try later".
Solve the problem with the mail server or change the administrator email (I think) .

Yes, it seems like the mail server, whether it's an SMTP server or the hosting's internal one, is limiting the number of messages you can send, as a form of anti SPAM I guess.