diff --git a/domain/mail-repository.js b/domain/mail-repository.js
index 2d51ef7..4b8a0c2 100644
--- a/domain/mail-repository.js
+++ b/domain/mail-repository.js
@@ -27,7 +27,7 @@ class MailRepository {
 	}
 
 	add(to, mailSummary) {
-		if (to !== 'undefined') {
+		if (to !== undefined) {
 			this.mailSummaries.set(to.toLowerCase(), mailSummary)
 		}
 	}