Assume that you create a signature that has an image by using Outlook on the web (formerly Outlook Web App) in a Microsoft Exchange Server 2016 environment. When you send an email to an external user from Outlook on the web, you may find that the image isn't visible in the signature. Instead, a cross mark will be shown in the signature. This issue may occur after you install the cumulative update 8 or a later update for Exchange Server 2016.
The issue occurs because the images that are inserted as data URI aren't promoted to inline images. Because the image promotion does not occur, most of the clients are not able to display the image.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.