diff --git a/options.go b/options.go
index bcf7d29..99e0ef4 100644
--- a/options.go
+++ b/options.go
@@ -103,6 +103,9 @@ func (o *Options) Validate() error {
 	if o.ClientSecret == "" {
 		msgs = append(msgs, "missing setting: client-secret")
 	}
+	if o.AuthenticatedEmailsFile == "" && len(o.EmailDomains) == 0 && o.HtpasswdFile == "" {
+		msgs = append(msgs, "missing setting for email validation: email-domain or authenticated-emails-file required.\n      use email-domain=* to authorize all email addresses")
+	}
 
 	o.redirectUrl, msgs = parseUrl(o.RedirectUrl, "redirect", msgs)