Hello MailPoet Team,

I am using MailPoet to send emails via a custom SMTP server (api.samotpravil.ru). My SMTP provider requires that I include the header Precedence: transaction in all outgoing emails to disable their local stop-list.

I have tried several approaches, but the header either doesn't get added or is overwritten. I need guidance on the correct MailPoet hook to use.

Environment details:

MailPoet version: last

WordPress version: 6.x

Sending method: SMTP (configured with host api.samotpravil.ru, port 1126)

What I've tried:

Using WordPress phpmailer_init hook — this works for regular wp_mail() but doesn't seem to fire for MailPoet's sending process.

Using MailPoet's mailpoet_mailer_smtp_options filter — I can see in the debug log that my function is called and I add the header to $options['custom_headers'], but the final email still shows Precedence: bulk. Here's the code I used:

php

add_filter('mailpoet_mailer_smtp_options', function($options) {

$options['custom_headers'] = ['Precedence:transaction'];

return $options;

});

Using mailpoet_mailer_smtp_transport_agent — this filter is marked as deprecated in your documentation, but I tried it anyway. It either doesn't fire or the header is still overwritten.

My questions:

What is the correct and currently recommended hook in MailPoet for adding custom SMTP headers (like Precedence) to all outgoing emails?

Is there a way to ensure that my custom header overrides any default header that MailPoet might be adding?

Could MailPoet be adding a Precedence: bulk header automatically? If so, how can I prevent this or replace it with my own?

If mailpoet_mailer_smtp_options is the right filter, why might my header not appear in the final email, even though it's present in the $options array?

Debug log entry (showing my filter is called):

text

[07-Mar-2026 11:51:51 UTC] === Added Precedence exactly as in support example ===

[07-Mar-2026 11:51:51 UTC] Header: Precedence:transaction

But the received email still shows:

text

Precedence: bulk

Any guidance would be greatly appreciated. I'm happy to provide more details or run additional tests if needed.

Thank you