Oct 3 13:01:01 zby postfix/smtp: 2B7905BEE3: host gmail-smtp-in.l.google.com[126.96.36.199] said: 421-4.7.0 [188.8.131.52] Our system has detected an unusual amount of 421-4.7.0 unsolicited mail originating from your IP address. To protect our 421-4.7.0 users from spam, mail sent from your IP address has been temporarily 421-4.7.0 blocked. Please visit http://www.google.com/mail/help/bulk_mail.html 421 4.7.0 to review our Bulk Email Senders Guidelines. i5si10046102mue.8 (in reply to end of DATA command)
Even though I did not notice anything suspicious it can be entirely possible that my computer was hacked and that someone is relying spam through it - but this message does not help me with solving that problem. And the linked page about rules for bulk senders only reminds me of the detailed procedures from 'The Trial' that never answered what really was the crime of Joseph K.