Outgoing newsletters result in 550 errors, but SMTP tests work fine

Hi there,

I’m encountering a strange issue that I’ve been unable to solve.

We have a CentOS 6.8 / WHM 58.0 Build 31 / exim 4.87-6 / PHP 5.6 server, and are using WordPress 4.6.1 with MailPoet 2.7.5

We have MailPoet configured to send via SMTP using an account on the server. This problem occurs regardless of whether we use port 25 or SSL / Port 465.

Whenever we run an SMTP test, the usual “SMTP works with MailPoet” comes through just fine, on both internal and external (@gmail.com, @yahoo.com, etc.) accounts.

HOWEVER, when we try to send a newsletter, whether via the “Send Preview” feature or batching out to one of the lists, EVERY SINGLE MESSAGE goes into our server’s mail queue and then bounces back with the following:

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

  mytestaccount@outlook.com
    host mx1.hotmail.com [207.46.8.167]
    SMTP error from remote mail server after end of data:
    550 Action not taken

Reporting-MTA: dns; subdomain.ourmailserver.com

Action: failed
Final-Recipient: rfc822;mytestaccount@outlook.com
Status: 5.0.0
Remote-MTA: dns; mx1.hotmail.com
Diagnostic-Code: smtp; 550 Action not taken

Sending e-mail out via Thunderbird or RoundCube webmail works just fine otherwise.

Anyone else ever run into such a problem before? Where should I start to try and solve this?

Any help or suggestions are appreciated – Thank you!