windows server 2008 - Suddenly Gmail and Hotmail block our emails

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:

Gmail

Failed Recipient: email@gmail.com
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

Hotmail

Failed Recipient:address@hotmail.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.

Thanks!

Answer

First of all check if your IP adress is listed in any blacklist - here, for example

Also, by the Hotmail response, it seems like one of computers in your network has virus sending spam. You can try to determine which one by monitoring your mail server (strange messages with lots of recepients, blank "from" field and so on) or by simply doing antivirus scan on all your computers. Also dont forget to change passwords for that user

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