Email formatting is differs with email platforms (Gmail, Outlook, Apple Mail, Mobile)

Email formatting is differs with email platforms (Gmail, Outlook, Apple Mail, Mobile)

The preview of the email looks fine, but when it is sent to different platforms, the some font sizes change. Also, on mobile the email, which is supposed to have 2 columns, condenses into one column. 

We would like the email to look uniform on all platforms. How do we fix this issue?