type MailRequest struct {
	From                 string   `protobuf:"bytes,1,opt,name=from,proto3" json:"from,omitempty"`
	To                   []string `protobuf:"bytes,2,rep,name=to,proto3" json:"to,omitempty"`
	Cc                   []string `protobuf:"bytes,3,rep,name=cc,proto3" json:"cc,omitempty"`
	Subject              string   `protobuf:"bytes,4,opt,name=subject,proto3" json:"subject,omitempty"`
	Body                 string   `protobuf:"bytes,5,opt,name=body,proto3" json:"body,omitempty"`
	Type                 string   `protobuf:"bytes,6,opt,name=type,proto3" json:"type,omitempty"`
	XXX_NoUnkeyedLiteral struct{} `json:"-"`
	XXX_unrecognized     []byte   `json:"-"`
	XXX_sizecache        int32    `json:"-"`
}

package main

import (
	"log"
	"net"
	"os"
	"time"

	"golang.org/x/net/context"
	"google.golang.org/grpc"
	"google.golang.org/grpc/reflection"

	"myMail/pb"

	gomail "gopkg.in/gomail.v2"
)

type server struct{}

var ch = make(chan *gomail.Message)

/*
Send is a simple function for send email
*/
func (s *server) Send(ctx context.Context, mail *pb.MailRequest) (*pb.MailStatus, error) {
	m := gomail.NewMessage()
	m.SetHeader("From", mail.From)
	m.SetHeader("To", mail.To...)
	m.SetHeader("Subject", mail.Subject)
	m.SetBody(mail.Type, mail.Body)
	ch <- m
	return &pb.MailStatus{Status: int32(0), Code: ""}, nil
}

func main() {
    // 監聽 50051 port
	lis, err := net.Listen("tcp", ":50051")
	if err != nil {
		log.Fatalf("無法監聽該埠口：%v", err)
	}
	s := grpc.NewServer()
	pb.RegisterMailServer(s, &server{})
	reflection.Register(s)
	go func() {
		d := gomail.NewDialer("smtp.gmail.com", 587, os.Getenv("GMAIL_ACC"), os.Getenv("GMAIL_PASS"))

		var s gomail.SendCloser
		var err error
		open := false
		for {
			select {
			case m, ok := <-ch:
				if !ok {
					return
				}
				if !open {
					if s, err = d.Dial(); err != nil {
						panic(err)
					}
					open = true
				}
				if err := gomail.Send(s, m); err != nil {
					log.Print(err)
				}
			// Close the connection to the SMTP server if no email was sent in
			// the last 30 seconds.
			case <-time.After(30 * time.Second):
				if open {
					if err := s.Close(); err != nil {
						panic(err)
					}
					open = false
				}
			}
		}
	}()
	if err := s.Serve(lis); err != nil {
		log.Fatalf("無法提供服務：%v", err)
		close(ch)
	}
}

package main

import (
	"context"
	"fmt"
	"log"
	"os"
	"net/http"

	"myMail/pb"

	"google.golang.org/grpc"
)

func main() {
	// 連線到遠端 gRPC 伺服器。
	conn, err := grpc.Dial("server:50051", grpc.WithInsecure())
	if err != nil {
		log.Fatalf("連線失敗：%v", err)
	}
	defer conn.Close()

	// 建立新的 Mail 客戶端，所以等一下就能夠使用 Mail 的所有方法。
	c := pb.NewMailClient(conn)

	// 傳送新請求到遠端 gRPC 伺服器 Mail 中，並呼叫 Send 函式
	mr := pb.MailRequest{
		From:    os.Getenv("MAIL_FROM"),
		To:      []string{"abc@example.com"},
		Cc:      []string{},
		Subject: "How to use gRPC",
		Body:    "Just done",
		Type:    "text/html",
	}
	http.HandleFunc("/send", func(w http.ResponseWriter, r *http.Request) {
		ret, err := c.Send(context.Background(), &mr)
		if err != nil {
			log.Fatalf("無法執行 Send 函式：%v", err)
		} else {
			fmt.Fprintf(w, "Send %s", ret.Code)
		}
	})

	log.Fatal(http.ListenAndServe(":8080", nil))
}

cat Dockerfile
FROM golang AS build-env
ARG BUILD_PATH
ADD . /go/src/myMail
RUN cd /go/src/myMail/$BUILD_PATH && go get && GOOS=linux GOARCH=amd64 CGO_ENABLED=0 go build -o app

FROM alpine
ARG BUILD_PATH
ARG EXPOSE_PROT
WORKDIR /app
COPY --from=build-env /go/src/myMail/$BUILD_PATH/app /app/
RUN apk update && apk add ca-certificates && rm -rf /var/cache/apk/*
EXPOSE ${EXPOSE_PROT}
# 50051, 8080