I just found one issue. It seems that when I test email template which is really really large (about 200kb, 1700 lines, more than 200000 characters) the preview screen from Outlook 2016 (Win7) and Outlook 2019 is blank. For all rest versions of Outlook in Litmus, all is fine.

If I test the same email template with EmailOnAcid, the preview screen is rendred fine in Outlook 2016 (Win7).

Any advices wht that is not working with Litmus?