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Our company emails are getting blocked (blacklisted) every other day. Can someone tell us why the emails are getting blocked. And how to rectify for being blocked. Your advise will be highly appreciated.

Gabor Lengyel
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syzh
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    You've probably ended up on a spam blacklist, which could be a result of a compromised server, or simply lots of recipients clicking "report spam" against your mails. Not enough information to be sure, and might not be a security question even then. – Matthew Nov 02 '16 at 10:12
  • Explain why the title of your question is "Exchange security". – dan Nov 02 '16 at 10:18
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    Please provide one anonymised reject message. – dan Nov 02 '16 at 10:18

1 Answers1

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Some usual reasons (not a comprehensive list):

  • Your mail server is an open relay, anybody on the internet can use it to send emails to any address. Spammers very quickly find these and use them to spread spam. This way your server will get blacklisted in RBLs (see below).

  • Your server is used to send spam by its legitimate clients due to malware on your internal client computers. Check the mail queues and see what emails go through. This will also likely result in the server getting into RBLs.

  • Your mail server is in a black list (RBL) for any reason. Some recipients check if the sender server is in a blacklist and will either not take emails from that server or automatically mark them as spam. You can check whether your server is blacklisted here or here or here among others.

  • You are sending email on behalf of other domains apart from your own (the apparent sender of the email is not your domain). You should look into things like SPF and DKIM, also here.

  • Your server is in a dynamic IP range, meaning your internet provider marked your IP range as dynamic. Some recipient servers either don't take emails from dynamic IP ranges or mark them as spam.

Gabor Lengyel
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