diff --git a/mail/mailer.go b/mail/mailer.go
index e34042878..43fafeb24 100644
--- a/mail/mailer.go
+++ b/mail/mailer.go
@@ -17,6 +17,7 @@ import (
 	"strconv"
 	"strings"
 	"time"
+	"unicode"
 
 	"github.com/letsencrypt/boulder/Godeps/_workspace/src/github.com/jmhodges/clock"
 )
@@ -52,6 +53,15 @@ type MailerImpl struct {
 	csprgSource idGenerator
 }
 
+func isASCII(str string) bool {
+	for _, r := range str {
+		if r > unicode.MaxASCII {
+			return false
+		}
+	}
+	return true
+}
+
 // New constructs a Mailer to represent an account on a particular mail
 // transfer agent.
 func New(server, port, username, password string) MailerImpl {
@@ -71,6 +81,9 @@ func (m *MailerImpl) generateMessage(to []string, subject, body string) ([]byte,
 	now := m.clk.Now().UTC()
 	addrs := []string{}
 	for _, a := range to {
+		if !isASCII(a) {
+			return nil, fmt.Errorf("Non-ASCII email address")
+		}
 		addrs = append(addrs, strconv.Quote(a))
 	}
 	headers := []string{
diff --git a/mail/mailer_test.go b/mail/mailer_test.go
index 2b09d7c3e..a0bc2226b 100644
--- a/mail/mailer_test.go
+++ b/mail/mailer_test.go
@@ -42,3 +42,10 @@ func TestGenerateMessage(t *testing.T) {
 	test.AssertEquals(t, fields[8], "")
 	test.AssertEquals(t, fields[9], "this is the body")
 }
+
+func TestFailNonASCIIAddress(t *testing.T) {
+	fc := clock.NewFake()
+	m := MailerImpl{From: "send@email.com", clk: fc, csprgSource: fakeSource{}}
+	_, err := m.generateMessage([]string{"遗憾@email.com"}, "test subject", "this is the body\n")
+	test.AssertError(t, err, "Allowed a non-ASCII to address incorrectly")
+}