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Starting from yesterday, all emails sent from our server are being rejected by Gmail and Hotmail.
Everything was working properly until now.
Here are the error messages received:
Failed Recipient: firstname.lastname@example.org Reason: Remote host said: 550 5.7.1 [server IP 1] Our system has detected an unusual rate of 5.7.1 unsolicited mail originating from your IP address. To protect our 5.7.1 users from spam, mail sent from your IP address has been blocked. 5.7.1 Please visit http://www.google.com/mail/help/bulk_mail.html to review 5.7.1 our Bulk Email Senders Guidelines. el7si1434447wib.69 - gsmtp
Failed Recipient:email@example.com Reason: Remote host said: 421 RP-001 (BAY0-MC3-F30) Unfortunately, some messages from [server IP] weren't sent. Please try again. We have limits for how many messages can be sent per hour and per day. You can also refer to http://mail.live.com/mail/troubleshooting.aspx#errors.
I do know the point of those messages - they both blocked our server because it might be sending out spam.
However, I don't understand what exactly could have caused this.
- Was someone spamming Gmail/Hotmail addresses from our server?
- Did some of the recipients report our emails as spam?
- Any other possibilities?
I'm honestly not sure where to look at first.