The following are known email display issues with Outlook, especially with Outlook 2016 and 2019 running on Windows.
These issues are not unique to emails sent from Envoke, the glitches are with Outlook itself.
Contacts viewing your emails in another version of Outlook or in a different email program aren't affected by these issues.
Background images aren't shown. A fallback background colour should be used. Read more.
Certain fonts won't show up as specified. A fallback font is used. Read more.
Thin lines are inserted in emails in Outlook 2016 on Windows. This is a known bug with Outlook 2016's old Microsoft Word based rendering engine.
Also in Outlook 2016 running on Windows, not all line heights are shown. For example line heights of 1.3 and 1.8 look the same.
Animated GIFs will only show the first frame of the animation. Check out this great blog post by Litmus for details.
Displaying transparent PNGs is not reliable in Outlook.
Additional details not specific to Outlook: