When a sign request email is sent, the "Review Document" button renders as a black-and-blue square without any text in the desktop Outlook client (tested on Version 2401 build 17231.20194, but widely repeatable across our company). Downloading Pictures (a common fix for Outlook formatting issues) does not fix this issue.

Opening the same email in Outlook for Web, Outlook Mobile, Gmail, and other email clients renders the Review Document button correctly. Competitor products all render their equivalent of the Review Document button correctly in all variants of Outlook.

Given the prevalence of Outlook in corporate environments, this makes documents sent via Box Sign look unnecessarily amateur.

More information and screenshots in Case # 3016302 - submitting this here at Box Support's suggestion.