In my opinion the Nextcloud Email App lacks a text-only view of emails. Any HTML code breaks the global layout esp. widths and creates security pains. When dealing with PGP mails, I cannot use them at all, because nothing is visible (I am using external tools for encyption/ decryption as recommended, PGP Applet).
So please please please dear Nextcloud Team fix your Email. Just make a text-only switch in the settings. This would be great!