diff --git a/email.go b/email.go
index 57d1b53..69b7932 100644
--- a/email.go
+++ b/email.go
@@ -301,7 +301,7 @@ func (e *Email) AttachFile(filename string) (a *Attachment, err error) {
 func (e *Email) msgHeaders() (textproto.MIMEHeader, error) {
 	res := make(textproto.MIMEHeader, len(e.Headers)+6)
 	if e.Headers != nil {
-		for _, h := range []string{"Reply-To", "To", "Cc", "From", "Subject", "Date", "Message-Id", "MIME-Version"} {
+		for _, h := range []string{"Reply-To", "To", "Cc", "From", "Subject", "Date", "Message-ID", "MIME-Version"} {
 			if v, ok := e.Headers[h]; ok {
 				res[h] = v
 			}
@@ -320,12 +320,12 @@ func (e *Email) msgHeaders() (textproto.MIMEHeader, error) {
 	if _, ok := res["Subject"]; !ok && e.Subject != "" {
 		res.Set("Subject", e.Subject)
 	}
-	if _, ok := res["Message-Id"]; !ok {
-		id, err := generateMessageID()
+	if _, ok := res["Message-ID"]; !ok {
+		ID, err := generateMessageID()
 		if err != nil {
 			return nil, err
 		}
-		res.Set("Message-Id", id)
+		res["Message-ID"] = []string{ID}
 	}
 	// Date and From are required headers.
 	if _, ok := res["From"]; !ok {
@@ -335,7 +335,7 @@ func (e *Email) msgHeaders() (textproto.MIMEHeader, error) {
 		res.Set("Date", time.Now().Format(time.RFC1123Z))
 	}
 	if _, ok := res["MIME-Version"]; !ok {
-		res.Set("MIME-Version", "1.0")
+		res["MIME-Version"] = []string{"1.0"}
 	}
 	for field, vals := range e.Headers {
 		if _, ok := res[field]; !ok {
diff --git a/pool.go b/pool.go
index 67f224a..75f75c5 100644
--- a/pool.go
+++ b/pool.go
@@ -8,7 +8,7 @@ import (
 	"net/mail"
 	"net/smtp"
 	"net/textproto"
-	"sync"
+	"sync/atomic"
 	"syscall"
 	"time"
 )
@@ -17,10 +17,9 @@ type Pool struct {
 	addr          string
 	auth          smtp.Auth
 	max           int
-	created       int
+	created       int32
 	clients       chan *client
 	rebuild       chan struct{}
-	mut           *sync.Mutex
 	lastBuildErr  *timestampedErr
 	closing       chan struct{}
 	tlsConfig     *tls.Config
@@ -52,7 +51,6 @@ func NewPool(address string, count int, auth smtp.Auth, opt_tlsConfig ...*tls.Co
 		clients: make(chan *client, count),
 		rebuild: make(chan struct{}),
 		closing: make(chan struct{}),
-		mut:     &sync.Mutex{},
 	}
 	if len(opt_tlsConfig) == 1 {
 		pool.tlsConfig = opt_tlsConfig[0]
@@ -84,7 +82,7 @@ func (p *Pool) get(timeout time.Duration) *client {
 	default:
 	}
 
-	if p.created < p.max {
+	if int(atomic.LoadInt32(&p.created)) < p.max {
 		p.makeOne()
 	}
 
@@ -142,24 +140,15 @@ func (p *Pool) replace(c *client) {
 }
 
 func (p *Pool) inc() bool {
-	if p.created >= p.max {
+	if int(atomic.LoadInt32(&p.created)) >= p.max {
 		return false
 	}
-
-	p.mut.Lock()
-	defer p.mut.Unlock()
-
-	if p.created >= p.max {
-		return false
-	}
-	p.created++
+	atomic.AddInt32(&p.created, 1)
 	return true
 }
 
 func (p *Pool) dec() {
-	p.mut.Lock()
-	p.created--
-	p.mut.Unlock()
+	atomic.AddInt32(&p.created, -1)
 
 	select {
 	case p.rebuild <- struct{}{}:
@@ -359,7 +348,7 @@ func addressLists(lists ...[]string) ([]string, error) {
 func (p *Pool) Close() {
 	close(p.closing)
 
-	for p.created > 0 {
+	for atomic.LoadInt32(&p.created) > 0 {
 		c := <-p.clients
 		c.Quit()
 		p.dec()