src/net/mail/message.go      | 23 +++++++++++++++++------
 src/net/mail/message_test.go | 11 +++++++++++

diff --git a/src/net/mail/message.go b/src/net/mail/message.go
index fbf1fca68f5e683b76d81a60e85e7e45330aa30d..80140a467771b99590c253eadfbc4e520dbc7a86 100644
--- a/src/net/mail/message.go
+++ b/src/net/mail/message.go
@@ -575,8 +575,10 @@ // consumePhrase parses the RFC 5322 phrase at the start of p.
 func (p *addrParser) consumePhrase() (phrase string, err error) {
 	debug.Printf("consumePhrase: [%s]", p.s)
 	// phrase = 1*word
-	var words []string
-	var isPrevEncoded bool
+	var (
+		words []string
+		sb    strings.Builder
+	)
 	for {
 		// obs-phrase allows CFWS after one word
 		if len(words) > 0 {
@@ -608,13 +610,22 @@ 		if err != nil {
 			break
 		}
 		debug.Printf("consumePhrase: consumed %q", word)
-		if isPrevEncoded && isEncoded {
-			words[len(words)-1] += word
-		} else {
+		switch {
+		case isEncoded:
+			sb.WriteString(word)
+		case !isEncoded && sb.Len() > 0:
+			words = append(words, sb.String())
+			sb.Reset()
+			words = append(words, word)
+		default:
 			words = append(words, word)
 		}
-		isPrevEncoded = isEncoded
 	}
+
+	if sb.Len() > 0 {
+		words = append(words, sb.String())
+	}
+
 	// Ignore any error if we got at least one word.
 	if err != nil && len(words) == 0 {
 		debug.Printf("consumePhrase: hit err: %v", err)
diff --git a/src/net/mail/message_test.go b/src/net/mail/message_test.go
index 3393b03af54bf256bd36c3dc2ed174843614b181..80a4b2da765e6a35b003b846bcfaf37c14ce1780 100644
--- a/src/net/mail/message_test.go
+++ b/src/net/mail/message_test.go
@@ -1262,6 +1262,17 @@ 		t.Errorf(`ParseAddressList("") = %v, %v, want nil, error`, list, err)
 	}
 }
 
+func BenchmarkConsumePhrase(b *testing.B) {
+	for _, n := range []int{10, 100, 1000, 10000} {
+		b.Run(fmt.Sprintf("words-%d", n), func(b *testing.B) {
+			input := strings.Repeat("=?utf-8?q?hello?= ", n) + "<user@example.com>"
+			for b.Loop() {
+				(&addrParser{s: input}).consumePhrase()
+			}
+		})
+	}
+}
+
 func BenchmarkConsumeComment(b *testing.B) {
 	for _, n := range []int{10, 100, 1000, 10000} {
 		b.Run(fmt.Sprintf("depth-%d", n), func(b *testing.B) {