Test-Newsletter (only) not sending after using SMPT

[User]

Hello support team,

I am experiencing a very specific issue with the test email function inside the newsletter editor.

Here is what we have set up and tested:

We are using Sender.net via SMTP for The Newsletter Plugin.

Sending a general test email from the main Newsletter SMTP settings works perfectly (the email is delivered).

However, when we create a new newsletter and use the “Send Test” button directly within the editor view, the test email never arrives (not even in the spam folder).

The system interface shows no errors when sending from the editor, but there are no delivery logs at Sender.net for these specific editor tests.

We have tried sending the test to different external email addresses (Gmail, etc.) and made sure the sender address matches the verified Sender.net domain.

Could you advise why the test email from the editor fails silently while the global SMTP test in your plugin works without issues? Are there specific overrides happening in the editor?

Thank you in advance for your help!
Best regards,
Katharina

[User]

I managed to attempt a real newsletter delivery, which finally triggered a specific error message that was hidden during the editor tests:

“SMTP Error: data not accepted. SMTP server error: DATA END command failed Detail: header Precedence is not allowed SMTP code: 550 Additional SMTP info: 5.6.0”

It appears that my hosting environment (WordPress.com) is automatically injecting a priority header via wpcomsh_smtp_add_priority_header. My SMTP provider (Sender.net) strictly forbids the “Precedence” header, which leads to the server rejecting the email data entirely at the end of the command.

I have already tried to remove the filter via a code snippet, but the host seems to re-inject it with a higher priority.

Does “The Newsletter Plugin” have an internal setting or a specific hook to force-strip these headers before they are passed to the SMTP relay?

[Michael]

Hello Katharins,

I’m speaking with our devs right now, I’ll get back to you as soon as possible.

Michael

[User]

Thank you, Michael. The problem is solved already! Stefano helped! 🙏🏻

[Michael]

Hello Katharina,

yes sorry about this, I just spoke with him : )

Michael

[Author]

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