Ran into this issue with a fresh composer require drupal/htmlmail.
Patch in comment #7 worked for me.
I am getting the following error:
TypeError: Drupal\htmlmail\Plugin\Mail\HtmlMailSystem::__construct(): Argument #10 ($mimeTypeGuesser) must be of type Symfony\Component\Mime\MimeTypesInterface, Drupal\Core\ProxyClass\File\MimeType\MimeTypeGuesser given, called in /var/www/html/docroot/modules/contrib/htmlmail/src/Plugin/Mail/HtmlMailSystem.php on line 147 in Drupal\htmlmail\Plugin\Mail\HtmlMailSystem->__construct() (line 109 of /var/www/html/docroot/modules/contrib/htmlmail/src/Plugin/Mail/HtmlMailSystem.php).
I am trying to supply a patch that will fix this issue.
Adding the core version requirement works. Can we get this in dev? Currently having to download manually.