Hi @kamilwrona,
Thanks for your feedback, we apologize for any inconvenience. Are you talking about this plugin : “https://fr.wordpress.org/plugins/newsletter/” ?
I just tried to install a WordPress website from scratch with :
Wordpress 5.4.1
Newsletter 6.6.3
SMTP Mailing Queue 1.2.0
Newsletter are sending correctly as HTML content, Content-Type is still text/html, even on web client receiving the newsletter email. Everything is fine on my test environment.
Could you verify if another plugin affect the behavior of SMTP Mailing Queue on your website ?
Do not hesitate to create a topic on support forum and we will dig together in order to understand what is happening on your side.
Yours Faithfully,
Birmania