Попробуйте использовать код из https://golang.org/pkg/net/smtp/#example_SendMail
package main
import (
"log"
"net/smtp"
)
func main() {
// Set up authentication information.
auth := smtp.PlainAuth("", "user@example.com", "password", "mail.example.com")
// Connect to the server, authenticate, set the sender and recipient,
// and send the email all in one step.
to := []string{"recipient@example.net"}
msg := []byte("To: recipient@example.net\r\n" +
"Subject: discount Gophers!\r\n" +
"\r\n" +
"This is the email body.\r\n")
err := smtp.SendMail("mail.example.com:25", auth, "sender@example.org", to, msg)
if err != nil {
log.Fatal(err)
}
}