Should be able to maintain .NET 4.0 compatibility. See comments on stackoverflow for more info.
I added this to a fork, please see that for ideas: https://github.com/HakanL/sendgridplus-csharp. It uses Task so it works in .NET 4.0.
Reopening until an asynchronous SMTP method is available as well
Closing this as SMTP concerns are being moved to sendgrid-smtpapi library