This started occurring after SP1 was applied to SharePoint 2013 and or IE11 was introduced to the Windows Environment on Windows 7 and Windows 10 devices.
If we send email using PowerShell as a test the same issue occurs. Html or Plain Text email body. As far as I am aware this is the same library SPD workflows and InfoPath use to send email see below. Not the full script below for brevity.
[Microsoft.SharePoint.Utilities.SPUtility]::SendEmail($web,0,0,$email,$subject,$body);
If I send using a PowerShell Script
using the Net.Mail.SmtpClient
typically used in C# .NET
to send emails Plain text or Html it works, same email body.