Golang smtp SendMail电子邮件组失败

I'm using Golang's smtp package and it's failing to send an email if the email is a Google email group containing 10 or so people in it. It works perfectly if I just email to a single gmail recipient. I have no idea where to start looking. Doesn't look like they have anything in the API documentation.

auth := smtp.PlainAuth(
    "",
    "erased for privacy reasons...",
    "erased for privacy reasons...",
    "smtp.gmail.com",
)
msg := []byte(
    "To: " + *email + "
" +
        "Subject: External health check failure!

" +
        "some stuff:

" + "
")
smtpErr := smtp.SendMail(
    "smtp.gmail.com:587",
    auth,
    "",
    []string{"erased for privacy reasons..."},
    msg,
)
if smtpErr != nil {
    fmt.Fprintf(os.Stderr, "Email alert failed with the following error: %v
", smtpErr)
}