In the Mail app on your Mac, choose Mail > Preferences, then click Signatures. If you selected multiple messages, each one is printed separately. Each email client has a default font if the font listed in the font-family stack is unavailable. Troubleshooting problems with emails and email signatures on mobile devices Problem: You experience various problems with signatures and email layout when you send emails from mobile devices. In the first beta of iOS 10, Apple Mail has a new mechanism that doesn't always auto-scale non responsive emails. You must press the keys before the black screen with the Apple logo appears.
Apple mail font problem I hope developer give some hint to everyone when users have issue with the tweaks.
