Hello @davelo ,
Thank you for reaching out to our Support team & sharing the details.
The Microsoft error 5.7.515 is being triggered because DKIM is failing (SPF=Pass, DKIM=Fail, DMARC=Pass). This doesn’t seems like an authentication/alignment issue, not a “bigger batch” issue.
Our SMTP plugin does not control newsletter batch size, unless you have the pro version.
Please check these items:
- DKIM record in DNS matches what your SMTP provider expects (Microsoft/Google/SendGrid/Mailgun, etc.).
- The email From address domain must match the authenticated domain
If you could share Diagnostic Test Report and which newsletter plugin you’re using, we’ll be able to assist you effectively..
Warm regards,
[ Signature deleted, don’t do that here, OK? ]
Today i’ve solved that issue by switching back to the SMTP plugin of Syed Balkhi. Microsoft does extra checks when it get’s +5K of e-mails. With this plugin, we got almost 100% failures on Microsoft adresses, i’m not sure if it’s a setting or an issue in your mailing header code. But with the SMTP plugin of Syed Balkhi, we almost have 100% delivery rate. (same -mail, same list).
I’m open for more investigation if you want (to share settings, etc), cause i like your plugin more then then one of Syed Balkhi (have logs and less banners). Only in the case, i had to switch back.
Hi, i did some research in why your plugin failed at sending in bulk to Microsoft servers, compared with the competitor.
The difference in the message headers is the message-id.
Yours:
“message-id”: “c14e7b2a9d0f3c6e8a1b5d7f2e9c0a3b@domain.com“,
Other plugin:
“message-id”: “c14e7b2a9d0f3c6e8a1b5d7f2e9c0a3b@mlg.domain.com“,
We use Mailgun and a subdomain: mlg.domain.com.
So, probably that is reason why Microsoft blocks on bulk mails, and i don’t see any plugin setting that’s adding that domain.com. So, it must be something in the code that takes the site domain, but would be better to use the domainname of mailservice (if used)
I hope this is helpful.
Hey @davelo ,
Thank you for sharing the update and for explaining what you observed in detail we really appreciate your openness to further investigation.
To clarify the situation from a technical standpoint:
This is the first time we’ve encountered this specific scenario, and we have not seen similar reports in the past. Based on Microsoft’s error response (5.7.515) and the diagnostic results you shared (SPF: Pass, DKIM: Fail, DMARC: Pass), the rejection is being triggered specifically due to DKIM failure, not due to email batch size or sending speed.
Microsoft does apply stricter checks for high-volume sending, but those checks still rely on authentication and alignment results. DKIM does not fail due to volume it fails when the signing domain, headers, or DNS configuration do not align exactly as expected by the receiving server.
We’d genuinely like to help debug it further if you could share the following:
- A Diagnostic Test Report generated while our plugin is active
- Name & version of the newsletter plugin being used
- A sample failed message header from Microsoft for comparison
This will allow us to identify whether there’s a subtle DKIM alignment or header compatibility issue and address it properly.
But if it’s an issue with DKIM, why does the other SMTP plugin has no issues, right?
An example of the mail header when it went wrong, i can’t access it anymore. The last submission via this plugin was saturday, and thoses logs aren’t visible in Mailgun (only last 5 days). Sorry.
All used SMTP plugins were the latest version.
The site currently uses: https://wordpress.org/plugins/wp-mail-smtp/. But as i said, we like to make that switch.
Can i sent the diagnostic report to a specific e-mail adddress, as it’s too public here.
Hey @davelo
You can send it over to [email protected], from Post SMTP Dashboard.
Hey @davelo
Could you please send over the Diagnostic test report?
Sorry for the delay, very busy.
I’ve sent now the diagnostic report.
Hi, i’ve sent almost 2 weeks ago that diagnostic report but i haven’t got a response. Or maybe i missed it (maybe in spam). But was there a follow up on this ticket?
Hey @davelo ,
Apologies for not getting back with you erlier. We did recieved the Diagnostic report & tried to reproduce the issue, we weren’t able to reprodce it.
However, if you comment out the following lines in PostmanMailgunMailEngine.php:
// add the messageId
$messageId = $message->getMessageId();
if ( ! empty( $messageId ) ) {
$this->addHeader( 'message-id', $messageId );
}
Then send email and check the Mailgun logs to compare the message ID.
Reference: PostmanMailgunMailEngine.php
I’m going to test it on staging next week. I’ll keep you informed.
Hello @davelo ,
We hope you are doing well.
We are marking this thread as resolved due to inactivity. If you encounter any further issues, please feel free to open a new thread.
Have a great day!
