Module Isn't Working or Emails Sent as Plain Text

As a 2nd test, I changed the 'Theme to render the emails' to "Claro" (which is the administration theme). After saving the changes and clearing all caches, I submitted another contact form entry.

Again, it's displaying only as plain text and breaking the lines as before.

Update: I've now removed this module and run some tests with only the Mail System module. And the e-mails still appear to be sent as plain text (i.e. the line breaks are still happening as before).

So perhaps this is more of a problem with the Mail System module. As such, I've also created an issue on their project page, and added it here as a related issue.

The Mail System maintainers helpfully replied, and made it clear that any such issues would be with this module. After that, I happened to (finally) stumble across and realize there is an additional GUI admin section for this module that I'd missed before...

So here's where things are at now:

1. At /admin/config/system/symfony-mailer-lite/transport, I added the 'Sendmail' Transport type and made it the default. I otherwise left all settings as is.

2. At /admin/config/symfony-mailer-lite/message-settings, I changed the 'Default Content Type' to "HTML". I left 'Always use the default content type' as unchecked. For 'Text format', I left that as "Plain text". For 'Plain Text Version', I left the option there as checked. And for 'Character Set', I left that as "UTF-8".

3. At /admin/config/system/symfony-mailer-lite/test, I then sent a test e-mail. It was received at an Outlook account, and says the module has been successfully configured. The text here also spans across the full width of the computer screen, so that looks great and I assume indicates it's sending as HTML

4. Next I went to the website's contact form and submitted it, but unfortunately I'm still seeing the same plain text line breaks on both e-mails. So I flushed all caches, and tried again. But to my dismay, I'm still seeing the e-mails as plain text. For what it's worth, the auto-reply e-mail is being sent to an account on mail.com, and the e-mail notification message is sent to a Gmail account.

Note also that I have set up SPF and DKIM for the website, and have already verified that these are in place with the contact form's e-mails.

I suppose the only other note is how things are set up currently on the Mail System page:

Default Mail System: Formatter and Sender are set to: Default PHP Mailer
Theme to render the emails: Current

Under 'Module-specific configuration', the following are set:
Module=Drupal Symfony Mailer Lite, Key=All, Formatter=Drupal Symfony Mailer Lite, Sender=Drupal Symfony Mailer Lite
Module=Contact, Key=All, Formatter=Drupal Symfony Mailer Lite, Sender=Drupal Symfony Mailer Lite

So it seems to me this is all set up correctly (assuming I want other system e-mails like password resets to remain handled with the default Drupal mail system as plain text, and the Contact form to use Drupal Symfony Mailer Lite as HTML).

I really don't know what else to try, if something is still set up incorrectly, or if there is a legitimate bug or issue with this module.

I will look forward to, and indeed expect, a reply from someone for this support request.

This module doesn't force a change to the content type of your email unless you check the box to always use the default content type.

The contact form must be sending a plain text email, and this module is dutifully passing it along in the specified content type. Check the box to always use the default, and it will force a change to the content type. However, note that this can lead to formatting issues. It will use the Plain Text text format to convert plain text to HTML, so you can adjust that format if you're unhappy with the results of the conversion.

Thanks so much for the reply, and that makes sense! So now at /admin/config/symfony-mailer-lite/message-settings, I checked the box for "Always use the default content type". And now, after doing some more tests, those 2 notifications e-mails are sending as HTML.

I appreciate also the note on the text formatting. Really, everything looks good to me by default *except* that the line breaks are missing. This is pushing sections of text together when there should be some spacing. For example, the plain text formatting in the e-mail notification looks like this:

Paul Rollins (not verified) (name@mail.com) sent a message using the
contact form at https://staging.domain.com/contact.

Message
Testing the e-mail setup again, after thinking I'd gotten it correct ...

But now, that message is formatted like this:

Paul Rollins (not verified) (name@mail.com) sent a message using the contact form at https://staging.domain.com/contact.
Message
Testing the e-mail setup again, after thinking I'd gotten it correct ...

As you can see, it'd look a lot better if there was a space (or line break) between the first sentence and the "Message" line of text.

So I'd like to do what you mentioned about adjusting that format if I'm unhappy with the results of the conversion. But I'm not sure where I would do this - is it in the UI (hopefully) or somewhere else? Also, if I just want to have the HTML follow the line breaks as they appear in the plain text, it'd be great to know what change(s) you'd recommend making to accomplish this.