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I have a DigitalOcean server which runs Ubuntu 14.04. I am having problems with sending emails from the PHP mail() function (which uses sendmail internally).

I think the issue may be related to my hosts file config. Here is what I have in /etc/hosts at the moment:

127.0.0.1 localhost localhost.localdomain ip-xxx-xxx-xxx-xxx

And in etc/hostname:

ip-xxx-xxx-xxx-xxx

(In the above two, I replaced the IP address digits with x)

Now I have a domain pointing to this server, let's call this mydomain.com.

So when my website mydomain.com sends an email, the email is going in to the junk mail folder. I ran a test on https://www.mail-tester.com and one of the issues it flags up is:

enter image description here

I have tried adding mydomain.com to the line above in the hosts file but this results in the email arriving after a long time or not arriving at all.

Here are the Received headers:

Received: from localhost.localdomain (unknown [xxx.xxx.xxx.xxx])
    (using TLSv1.2 with cipher xxx (256/256 bits))
    (No client certificate requested)
    by mail-tester.com (Postfix) with ESMTPS id xxx
    for <test-xxx@mail-tester.com>; Mon, 29 Apr 2019 18:35:18 +0200 (CEST)

Received: from localhost.localdomain (localhost [127.0.0.1])
    by localhost.localdomain (8.15.2/8.15.2/Debian-3) with ESMTP id xxx
    for <test-xxx@mail-tester.com>; Mon, 29 Apr 2019 17:35:18 +0100

Received: from mydomain.com (www-data@localhost)
    by localhost.localdomain (8.15.2/8.15.2/Submit) with SMTP id xxx
    for <test-xxx@mail-tester.com>; Mon, 29 Apr 2019 17:35:18 +0100

How to fix?

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When a mail is sent via SMTP, the connecting server identifies itself via the HELO command. See https://en.wikipedia.org/wiki/Simple_Mail_Transfer_Protocol#SMTP_transport_example

You need to change this in the configuration settings for the smtp client. For sendmail specifically, there's an example here: https://serverfault.com/questions/205271/how-to-specify-outgoing-helo-with-sendmail

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