From my Plesk managed server I keep receiving emails addressed to the configured administrator (me). Nothing special there. These mails are sent from an empty sender with no body, no subject nor any other relevant headers.

I have changed the server administrator's e-mail address to an alternate one which is then the recipient.

The mails are sent at irregular intervals. With variations in the timestamp I get an email every 30 minutes. Then there's a break and two and a half hours later I get the same email.

I already have turned of crond for more than a day to see if this is the source of the problem. On that server there is no "foreign" software running which would cause that behavior.

I have no more guesses as to why this is happening. Any suggestions?