Support of 3rd type / custom email tempaltes

I went ahead and modified a few lines in your plugin so it can support integration with other email, ( since your plugin is great, but misses just a bit to be awsome! )

my request if you find it proper and useful integrate it into your base code, so future versions will include it (since there are no other hooks that I can use to obtain this currently )

so, the intention is to use wp_mail, and add template: header for the plugin to use the header

add_filter( 'emtmpl_user_emails', function( $emails ) {
	$emails[ 'my_custom_template' ] = esc_html__( 'This is my custom template', 'domain' );
	return( $emails );
} );

wp_mail( 'johndoe@domain.com','{site_title} This is our email subject with shortcodes', 'Content here but it doesnt matter since we use the template', [ 'template: my_custom_template' ] );

for that i had modified a few lines in your email-trigger.php file in the use_default_email function

this is the adjusted function code ( around lines 515 ), marked the new code in START: END

public function use_default_email( $args ) {
		$use_default_tmpl = false;

		if ( ! $this->rendered ) {
			if ( $this->check_ignore( $args['message'] ) ) {
				$args['message'] = str_replace( $this->ignore_key, '', $args['message'] );

				return $args;
			}
			
			// START: Adding Support for dynamic template choosing from wp_mail via headers
			$headers = is_array( $args[ 'headers' ] ) ? $args[ 'headers' ] : explode( "\n", str_replace( "\r\n", "\n", $args[ 'headers' ] ) );
			foreach( $headers as $header ) {
				list( $name, $content ) = explode( ':', trim( $header ), 2 );
				if ( strtolower( trim( $name ) ) == 'template' ) {
					$template = strtolower( trim( $content ) );
					break;
				}
			}
			
			$template = $this->find_template( isset( $template ) && $template ? $template : 'default' );
			// $template = $this->find_template( 'default' );
			// END
			
			if ( ! empty( $template ) ) {
				$message = $this->render_email_html( $template->ID, $args['message'] );

				if ( $message ) {
					$this->rendered   = true;
					$use_default_tmpl = true;
					$args['message']  = $message;
				}
			}
		}

		if ( $this->rendered ) {
			// START: wrapped shortcodes with filter in order to provide support for additional shortcodes
			$re              = apply_filters( 'emtmpl_common_shortcodes', Utils::common_shortcodes() );
			// END
			$args['message'] = str_replace( array_keys( $re ), array_values( $re ), $args['message'] );
			$args['message'] = Utils::minify_html( $args['message'] );
			
			// START: Changed to support replacing of shortcodes inside subject, since we already are adding support for filters on shortcodes
			//if ( ! $use_default_tmpl ) { // Disabled if so it can support changing of shortcodes always
				$args['subject'] = str_replace( array_keys( $re ), array_values( $re ), $this->email_subject );
			//}
			// END
			
			add_filter( 'wp_mail_content_type', [ $this, 'replace_content_type' ] );
		}

		$this->rendered = false; //Reset

		return $args;
	}