github.com/agext/email (go): Package email provides a fluid API for composing and sending email messages.  Any application needs one (and usually only one) Sender, representing an SMTP account connection information plus a sender Address.  A Message contains all the information required to create an email message. Usually, at least some parts of each message are templates to be filled with data from the rest of the application. Also, it is often convenient to define base messages on program initialization, and clone them for fine-tuning and send-out when needed.  Below is an overly-simplified example, to showcase the basic functionality. Note that by not defining From: and To: addresses for a message, they default to the sender address, which is convenient for system messaging.
Keywords: email|email-composer|email-sender|email-template|mailer|smtp-mail