smtp/server: return an error if server does not provide certificate

Certificate is a mandatory requirement to running the server.  It used
to open port for mail submission.

This change return an error instead of nil when TLS certificate is not
defined.

1 files changed, 1 insertions, 1 deletions

diff --git a/lib/smtp/server.go b/lib/smtp/server.go
index dd8d5cd2..61687978 100644
--- a/lib/smtp/server.go
+++ b/lib/smtp/server.go
@@ -562,7 +562,7 @@ func (srv *Server) initListener() (err error) {
 	}
 
 	if srv.TLSCert == nil {
-		return nil
+		return fmt.Errorf("smtp: server certificate is not defined")
 	}
 
 	tlsCfg := &tls.Config{