Mail rate exceeded limit

Mail rate exceeded limit

We've just received this:

Dear User,

Correo saliente bloqueado. Motivo:Mail rate exceeded limit

However, we realize that this might have been unintentional. To unblock, please  click here .

Thanks,
Zoho Mail Team

P.S.: This is an auto-generated message. Please do not reply to this email.

I have checked the mail accounts which usually send mail and haven't found any abnormal sending rate. However, we would like to know the culprit to investigate if any user has been compromised.