We've always wanted to build a programmatic email server for developers. To be fair we were a bit naive when started and had no idea what was ahead of us. We knew that sending was hard, but we've no idea how hard and messy it actually is.
That said, in my view, that's the entire point of your service. Every language has an SMTP library; the hard part - the thing I'm paying you for - is the constant cat and mouse.