Most mail uses what’s called MIME-encoded multipart mail with most clients sending both HTML formatted messages and plain text.
Quit mail and restart, and test again. See: Close an app on your iPhone or iPod touch - Apple Support
Failing that... Restart your iPhone, and try viewing the mail again. See Restart your iPhone - Apple Support
Failing that... If you have an add-on VPN client, or add-on ad-blocking, or add-on anti-malware installed on your iPhone, remove it, and restart, and try again.
Is this one mail server, or—if you have several mail accounts configured—more than one server? If this is just one mail server, see if the mail renders correctly in the web mail interface that most vendors provide.
If the above doesn’t work, stop back again with an update—if the above don’t sort this, then this case might be heading toward a reset and reload, but that wouldn’t be my initial course here.