Email::render() generates DBHTMLFragment instead of string for plain-text part of email

[kinglozzer]

When you specify a template for the plain-text part of an email, Email::render() generates a DBHTMLFragment instance instead of a string for that part.

Affected Version

4.4.1, probably earlier

Description

silverstripe-framework/src/Control/Email/Email.php

Lines 796 to 799 in e1afcf7

	// Render plain part
	if ($plainTemplate && !$plainPart) {
	$plainPart = $this->renderWith($plainTemplate, $this->getData());
	}

This generates a DBHTMLFragment object, which is then passed to the Swift message via $this->getSwiftMessage()->addPart($plainPart, 'text/plain', 'utf-8');. addPart() is documented as accepting string|Swift_OutputByteStream, which DBHTMLFragment isn’t.

For the Postmark driver, this results in an empty plain-text part which means that the email is discarded by their API.

Just tacking ->Plain() on the end fixes it