diff --git a/src/mail.js b/src/mail.js
index 1d32ac67c..8e5ba2e33 100644
--- a/src/mail.js
+++ b/src/mail.js
@@ -72,6 +72,7 @@ function MailError(reason, errorOrMessage) {
 util.inherits(MailError, Error);
 MailError.INTERNAL_ERROR = 'Internal Error';
 MailError.BAD_FIELD = 'Bad Field';
+MailError.ALREADY_EXISTS = 'Already Exists';
 MailError.NOT_FOUND = 'Not Found';
 
 function checkOutboundPort25(callback) {