Configuring Postfix as email relay for Office 365

Posted: April 11, 2018 in Linux

Install sasld

yum install cyrus-sasl cyrus-sasl-plain cyrus-sasl-md5
systemctl start saslauthd

in /etc/postfix folder create file sasl_passwd and put username and password of mailbox which will be used as relay


To set default “from” to be this email open file /etc/postfix/generic

Add this at bottom (this is amazon AWS instance)

All emails will be rewritten with in from field with one we just specified

postmap hash:/etc/postfix/sasl_passwd
postmap hash:/etc/postfix/generic

now add following lines in /etc/postfix/

relayhost = []:587
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_auth_enable = yes
smtp_generic_maps = hash:/etc/postfix/generic
smtp_tls_security_level = may
smtp_sasl_security_options = noanonymous

Restart postfix service and you should be able to send emails through Office 365

If you get “Office 365 unreachable” error, in /etc/postfix/
change inet_protocols line from all to ipv4

Certificate authentication

If we want to secure our connection we can secure it with certificate:

openssl s_client -showcerts -starttls smtp -crlf -connect

In my case i got 2 certificates, i copied it in cacert.pem file


then add following lines to /etc/postfix/

smtp_send_dummy_mail_auth = yes
smtp_always_send_ehlo = yes
smtp_tls_security_level = encrypt
smtp_use_tls = yes
smtp_tls_CAfile = /etc/postfix/cacert.pem

Restart postfix service and your traffic is now encrypted

  1. Jordan Acosta says:

    Muchas gracias, funciono perfecto con esta documentacion.


  2. Mikhail says:

    Great instruction! helped me a lot!


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