I'm a novice at Go, but I've written several email libraries in other languages. Two lessons from that experience that I thought I'd pass on: - I've found it

I'm a novice at Go, but I've written several email libraries in other languages. Two lessons from that experience that I thought I'd pass on: - I've found it a more flexible architecture to separate the send functionality off from your email object itself and into its own service class. So instead of doing email.Send(), you do emailService.Send(email). The content of an email, and how to send that content, are separate responsibilities. Separating the two also allows for alternate implementations of the Send functionality: one (like here) that does a synchronous SMTP transfer, one that might drop the message into an internal queue so that client code doesn't need to wait for the (potentially fairly slow) SMTP transfer, one that simply appends to a local file for debugging, etc. (These are all examples from real apps.) - Have you considered making required properties (to, from, subject) properties of the constructor? Doing so makes guaranteeing that they're set a compile-time check rather than a runtime check, which can make a library more robust and suss out bugs earlier.