fix some var ref errors

go.mod

@@ -59,7 +59,6 @@ require (
 	golang.org/x/crypto v0.0.0-20190403202508-8e1b8d32e692
 	golang.org/x/net v0.0.0-20190522155817-f3200d17e092
 	golang.org/x/sys v0.0.0-20190403152447-81d4e9dc473e
-	google.golang.org/appengine v1.4.0 // indirect
 	google.golang.org/genproto v0.0.0-20190401181712-f467c93bbac2
 	google.golang.org/grpc v1.21.0
 	gotest.tools v2.2.0+incompatible // indirect

pkg/notifications/email.go

@@ -64,10 +64,12 @@ func newEmailNotifier(c *cobra.Command, acceptedLogLevels []log.Level) t.Notifie
 }
 
 func (e *emailTypeNotifier) buildMessage(entries []*log.Entry) []byte {
-	if SubjectTag == "" {
-		emailSubject := "Watchtower updates"
+	var emailSubject string
+
+	if e.SubjectTag == "" {
+		emailSubject = "Watchtower updates"
 	} else {
-		emailSubject := SubjectTag + " Watchtower updates"
+		emailSubject = e.SubjectTag + " Watchtower updates"
 	}
 	if hostname, err := os.Hostname(); err == nil {
 		emailSubject += " on " + hostname