550未采取请求的操作：邮箱不可用

Question

我正在使用gopkg.in/gomail.v2在golang平台上实现电子邮件发送功能。 所以我使用mail.com作为发送电子邮件地址。

dailer := gomail.NewPlainDialer(mail_info.MailServer, mail_info.MailPort, mail_info.MailUser, mail_info.MailPassword)
sender, err := dailer.Dial()

if err != nil {
    Mailer = nil
    fmt.Println("mail setting have been failed!")
    return false
}

err = Mailer.Send(mailer.Email{
    TO:      []mail.Address{{Address: "appmail@app.com"}},
    From:    &mail.Address{Address: payload.Email},
    Subject: "Question",
    Text:    "From User",
    HTML: "<p>email : " + payload.Email + "</p>" +
        "<p>location : " + payload.Location + "</p>" +
        "<p>feedback : " + payload.Feedback + "</p>",
})

使用新的发送电子邮件地址，我多次发送邮件。 之后，我收到了错误消息。

"gomail: could not send email 1: 550 Requested action not taken: mailbox unavailable"

我不确定究竟是什么原因，我该如何解决？ 我希望能帮助任何人解决我的问题。感谢。