diff --git a/internal/dovecot/dovecot.go b/internal/dovecot/dovecot.go
index 9a7f26e..855cfee 100644
--- a/internal/dovecot/dovecot.go
+++ b/internal/dovecot/dovecot.go
@@ -118,7 +118,7 @@ func (a *Auth) Exists(user string) (bool, error) {
 	resp, err := conn.ReadLine()
 	if err != nil {
 		return false, fmt.Errorf("error receiving response: %v", err)
-	} else if strings.HasPrefix(resp, "USER\t1\t"+user+"\t") {
+	} else if strings.HasPrefix(resp, "USER\t1\t") {
 		return true, nil
 	} else if strings.HasPrefix(resp, "NOTFOUND\t") {
 		return false, nil
diff --git a/test/t-11-dovecot/config/dovecot.conf.in b/test/t-11-dovecot/config/dovecot.conf.in
index d770287..3f59a86 100644
--- a/test/t-11-dovecot/config/dovecot.conf.in
+++ b/test/t-11-dovecot/config/dovecot.conf.in
@@ -5,6 +5,10 @@ ssl = no
 default_internal_user = $USER
 default_login_user = $USER
 
+# Before auth checks, rename "u@d" to "u-AT-d". This exercises that chasquid
+# handles well the case where the returned user information does not match the
+# requested user.
+auth_username_format = "%n-AT-%d"
 
 passdb {
 	driver = passwd-file
diff --git a/test/t-11-dovecot/config/passwd b/test/t-11-dovecot/config/passwd
index 0182c1a..2ceb0ee 100644
--- a/test/t-11-dovecot/config/passwd
+++ b/test/t-11-dovecot/config/passwd
@@ -1 +1 @@
-user@srv:{plain}password:1000:1000::/home/user
+user-AT-srv:{plain}password:1000:1000::/home/user