Every mail application like Outlook only supports a limited set of HTML. HTML elements that are not supported are usually ignored or create unwanted visual results. That is why it is important to choose HTML elements supported across as many Mail applications as possible. Here are some resources that could prove helpful:https://caniuse.email/https://www.sitepoint.com/rules-best-practice-email-design-coding-practices/https://www.smashingmagazine.com/2021/04/complete-guide-html-email-templates-tools/A general rule of thumbUse ...Text (with links)Images (with simple links, no link maps!)TablesBackgrounds in tables only... and nothing else to design your mail signature. If you need to use styles, then use inline styles and no stylesheets.Elements like 'div' or style properties like 'margin' might not render at all or not consistently across mail applications. You could even encounter differences in rendering within the same product like Outlook for Windows and Outlook for the Browser!Please let us know if you come across another valuable resource that would help you design your mail content like mail signatures that work across as many mail apps as possible.