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.
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.