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I have an installation of iRedMail version 0.9.7 running on Scientific Linux (RHEL) 7 :

Linux smtp.fabricobjects.com 3.10.0-862.6.3.el7.x86_64 #1 SMP Tue Jun 26 12:13:22 CDT 2018 x86_64 x86_64 x86_64 GNU/Linux

Postfix can send mail to mail-tester.com and get a perfect score of 10 out of 10 - but when I send email to specific email addresses at outlook.com or gmail.com I get an error in /var/log/maillog:

# cat /var/log/maillog |grep to |less

Jul 23 17:30:05 smtp postfix/error[20110]: 111F73078F82: to=<fabricobjects@outlook.com>, relay=none, delay=374, delays=374/0.03/0/0.02, dsn=4.3.0, status=deferred (unknown mail transport error)

I am using let's encrypt certificates that I manually obtained from an online website like freessl.

The certificates work very well for the iRedMail admin page and roundcube webmail.

I also am using the RHEL installation configured Selinux U.S Government Servers Policy that you can enable during the install process.

I can receive emails from outlook.com and gmail.com but replies do not make it back to those addresses.

I don't understand why I got a perfect 10 out of 10 score on mail.tester.com but can't send or reply to outlook.com or gmail.com

  • that log ain't clear... can you crank up the log level in postfix or try a specific tool like unicom.com/sw/rlytest to see exactly what is happening on the SMTP session? – thrig Jul 23 '18 at 23:50
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This might be a case of Greylisting, a popular anti-spam mechanism.

The recieving server initially rejects the message. After a while, the sending MTA should then resubmit the message which will then be accepted.

You don't have to do anything, postfix will automatically resubmit the message.

References:

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