diff --git a/plinth/modules/email/data/usr/share/freedombox/etc/dovecot/conf.d/2.4/15-freedombox-auth.conf b/plinth/modules/email/data/usr/share/freedombox/etc/dovecot/conf.d/2.4/15-freedombox-auth.conf
index 655de1de6..d81b6ac9c 100644
--- a/plinth/modules/email/data/usr/share/freedombox/etc/dovecot/conf.d/2.4/15-freedombox-auth.conf
+++ b/plinth/modules/email/data/usr/share/freedombox/etc/dovecot/conf.d/2.4/15-freedombox-auth.conf
@@ -7,4 +7,4 @@
 # Outlook and Windows Mail work only with LOGIN mechanism, not the standard PLAIN
 auth_mechanisms = plain login
 
-auth_username_format = %{user | lower}
+auth_username_format = %{user | username | lower}