This CentOS 7.0 machine is a replacement for the old machine which was running CentOS 5.11. I was not having this problem on the 5.11 machine.
Computers on my local network running Windows 7 and send e-mail using Thunderbird. Thunderbird is configured to use my local network host as the SMTP server.
When I try to send an e-mail from Thunderbird on one of these local PCs to a recipient at gmail.com, the e-mail is rejected with the following error:
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... while talking to gmail-smtp-in.l.google.com.: >>> >>> DATA <<< 550-5.7.1 [2606:400:0:5:240:f4ff:fe04:c83c 12] Our system has detected that <<< 550-5.7.1 this message is likely unsolicited mail. To reduce the amount of spam <<< 550-5.7.1 sent to Gmail, this message has been blocked. Please visit <<< 550-5.7.1 http://support.google.com/mail/bin/answer.py?hl=en&answer=188131 for <<< 550 5.7.1 more information. g6si67799938pat.30 - gsmtp 554 5.0.0 Service unavailable
In trying to debug this, I tried connecting to the gmail server directly (using telnet on port 25) and then doing the normal protocol (EHLO ..., MAIL FROM: ..., RCPT TO: ..., etc.) and this works fine. The message is accepted for delivery and is in fact delivered).
I also tried sending a message to the same recipient using a perl script running on the same Windows 7 PC. The perl script utilized the Net::SMTP library and uses my CentOS 7.0 host as the mail server). This message, too, is accepted for delivery and is then delivered successfully.
On the CentOS 7.0 host, I'm running sendmail 8.14.7, and I have it configured to allow relay for the hosts on my local network (if you want me to show more of my sendmail configuration, I can but I think the above "perl" test shows that it is allowing this properly).
Can anyone please suggest what I can try next?