Complain about malformed email addresses

src/freedombone-app-keyserver

@@ -390,6 +390,10 @@ function keyserver_sync {
                     echo "$other_keyserver_email" >> /etc/sks/mailsync
                     chown -Rc debian-sks: /etc/sks/mailsync
                 fi
+            else
+                dialog --title $"Sync with other keyserver" \
+                       --msgbox $"Email doesn't look right: $other_keyserver_email" 6 60
+                return
             fi
         fi
     fi