diff --git a/core/dovecot/conf/dovecot.conf b/core/dovecot/conf/dovecot.conf
index a9ec2676..bc5055d9 100644
--- a/core/dovecot/conf/dovecot.conf
+++ b/core/dovecot/conf/dovecot.conf
@@ -64,6 +64,7 @@ plugin {
 ###############
 # Authentication
 ###############
+auth_username_chars =
 auth_mechanisms = plain login
 disable_plaintext_auth = no