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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:

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!

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marked as duplicate by Falcon Momot, RobM, Greg Askew, mdpc, Ward Aug 25 '13 at 4:47

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

1  
Does your server have a dynamic ip? –  ott-- Aug 24 '13 at 19:11
    
@ott the server has a static IP address –  Ivanhoe123 Aug 24 '13 at 19:15
    
Have you found something in your outgoing mail log? The most obvious way would be sending spam from a script/program from a rootkit tho. –  ott-- Aug 24 '13 at 20:30

1 Answer 1

up vote 1 down vote accepted

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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Thanks a lot for your answer; will run the domains/IP addresses through the blacklist checks and start the AV scan. Will post back as soon as it's done. –  Ivanhoe123 Aug 24 '13 at 19:16
    
Just did a spamcheck on several websites - none of the server IP addresses are blacklisted. –  Ivanhoe123 Aug 24 '13 at 19:23
    
Keep in mind that passwords may be already stolen and some remote computer may be using your mail server simply as "proxy" for sending spam, so monitoring server and reading logs is more preferrable –  strange walker Aug 24 '13 at 19:27

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