Nice.

*Reads. Ponders.*

Was that in the admin dashboard or via email?

E-mail! For several minutes all outgoing platform mails contained the plugin’s design message. Including admin emails sent to me with customer purchase info, and emails to customer that should contain their purchase info…

That shouldn’t be included in the email. Just on the customizer preview email.
Just to confirm , on which email you got that?

Just to confirm, I tested the latest version and I’m not getting that error.

The code of our plugin works like this:

if( is_customize_preview() ) {
	include_once( apply_filters('mailtpl/customizer_template_message','default-message.php'));
} else {
	echo '%%MAILCONTENT%%';

If you are in the customizer you get the preview message which includes that phrase, but If not, the original message it’s sent.
I don’t see a way of those two message (original and preview) mixing . Also you just pasted a phrase . The whole message looks like this:

`Here you will see the email content that is normally sent in WordPress.

The email template is responsive an fully customizable. I hope you enjoy it!

We would like to ask you a little favour. If you are happy with the plugin and can take a minute please leave a nice review on WordPress. It will be a tremendous help for us!

Placeholders
You can use any of these placeholders in your emails content or templates and they will be automatically replaced

%%BLOG_URL%%
%%HOME_URL%%
%%BLOG_NAME%%
%%BLOG_DESCRIPTION%%
%%ADMIN_EMAIL%%
%%DATE%%
%%TIME%%
%%USER_EMAIL%% (not on sendgrid)`

Did you get the whole thing ?

SO, again, what happened today is that for a while (several minutes?), outgoing mails from my WordPress installation (Contactform 7, LearnDash LMS) would contain not only the HEADER design as set through the Email Template, but ALSO the default preview text that you would usually see in the design editor preview. User case: a customer sent in a COntactForm 7 submission and BOTH the customer (confirmation email, in Gmail) and I as admin (submission details, Outlook) received the emails with usual email title, however, with the entire email body text gone and replaced with that preview text. Note that I also use the Sendinblue plugin for dedicated SMTP.

Somehow, that design preview text found its way into real outgoing emails! I cannot attached a screenshot, or else I’d include a copy of one of those mails.

Adding: and yes, those placeholders were also in there, where the code was replaced with the corresponding actual values: the site URL, the site title, etc. Etc.

%%BLOG_URL%%
%%HOME_URL%%
%%BLOG_NAME%%
%%BLOG_DESCRIPTION%%
%%ADMIN_EMAIL%%
%%DATE%%
%%TIME%%
%%USER_EMAIL%% (not on sendgrid)

Thanks for the clarification. On your first post, I understood you were getting the original email and below the preview message.

It seems something on your end it’s setting is_customize_preview() to true. Did you run updates recently?

To be honest I’m not sure what could be causing the bug. It’s also weird that it happened for a while and not anymore.
I would recommend updating ally our plugins and in case it happens again, try deactivate them to see if any is generating the issue.