So I ran into an issue where the email format would be altered once received on an Gmail or Yahoo Account. Additionally the signatures were being attached via “WinMail.Dat” . The Exchange server was 2013 and once the email was received at Gmail or Yahoo the font would be changed from bolded, colored, underlined, style, etc… to a generic font. This was obviously an issue so after investigation here is the fix.

You need to Disable TNEF on remote domains. Below is the command to run in EMS (Exchange Management Shell) for a 2k13 Exchange server

Get-RemoteDomain | Set-RemoteDomain -TNEFEnabled $False

Once the command is run you then need to restart the Exchange Transport Service.