Problem/Motivation
In \Drupal\Core\Mail\Plugin\Mail\PhpMail::format() there is a conversion from HTML to text which occurs without checking if the input is an instance of MarkupInterface. This causes corruption of a mail that was in fact plain text and contains text that resembles HTML.
E.g.
Today I learnt Greek letters alphaβ they are hard to write.
In HTML, ampersand must be written as &.
I saw your house and <wow> it is great.
If a<b and b<c then a<c.Steps to reproduce
Install a contrib module that allows sending of messages in plain text format. An example is simplenews. Type a message like the example above.
Proposed resolution
Only call htmlToText() if the input implements MarkupInterface.
Remaining tasks
None.
User interface changes
None.
API changes
None.
Data model changes
None.
Release notes snippet
None.