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Getting 550 5.7.1 Message rejected as spam by Content Filtering when sending a test email via telnet to a customers on-prem Exchange 2010 server. If I send them an email via Outlook, it gets an SCL of 5 and goes into the users Junk E-mail folder - it seems like this is due to the HTML signature we append to all outgoing emails, if we remove the HTML signature we get an SCL of 0 or 1.

I don't understand why a telnat would get that response.

Any ideas?

Thanks

JBastyan
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    That is difficult to say without a transcript of your telnet session... – HBruijn Aug 15 '17 at 15:45
  • It's a simple telnet session with the data being . – JBastyan Aug 16 '17 at 07:32
  • So you're sending an smtp message with only an envelope but without a body and are therefore also missing the typical/mandatory headers such a `Date:` and `Subject:` - yes that would easily qualify your message as malformed and spam – HBruijn Aug 16 '17 at 07:44
  • When I have sent a test email via telnet, I have never specified Date: or Subject: and always been accepted. I have just done another test with Date and Subject, and still getting the same message. 550 5.7.1 Message rejected as spam by Content Filtering – JBastyan Aug 16 '17 at 13:17
  • OK, just tested this again (I think I might have had a few typos the first time). I can get a message queued for delivery but the SCL level is 5 and thus gone into the Junk Email folder and I don't know why. I have never had to specify Date and Subject for example, perhaps Microsoft have tightened something up in an update? – JBastyan Aug 16 '17 at 14:12
  • I don't know very much about native spam filtering support in Exchange, and your customer may also have an add-on installed or is using anti-virus/spam filtering solution that may be quite strict. A requirement that for instance the SMTP command from the initial `MAIL FROM: My Name ` command should match with the `From: ` header used in the DATA section with the actual e-mail message might be quite valid and/or effective. I would simply get the source, including all headers, of a message that was successfully delivered, modify as needed and copy-and-paste that as DATA – HBruijn Aug 16 '17 at 15:04
  • As far as I can tell, the Exchange Content Filter agent is giving this telnet email a score of 5. They have an external Anti-Spam filter but telnet makes a direct connection to the server and thus bypasses this. I have checked out public IP against blacklists, it's not on any - I just don't understand what could be causing a telnet email to be SCL 5. – JBastyan Aug 17 '17 at 07:42
  • This doesn't surprise me. A simple telnet test would be a very basic content stream and therefore is likely to get the message flagged as spam. I don't use telnet for anything more than message processing testing. Furthermore, the content filter is now largely useless as Microsoft no longer provide updates for it. Therefore I would suggest disabling it and using a third party tool instead. – Sembee Aug 17 '17 at 11:56

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