diff --git a/lib/net/smtp.rb b/lib/net/smtp.rb
index cca06e6..ff2019c 100644
--- a/lib/net/smtp.rb
+++ b/lib/net/smtp.rb
@@ -668,6 +668,7 @@ def do_start(helo_domain, user, secret, authtype)
       if user || secret || authtype
         check_auth_args authtype, user, secret
       end
+      @capabilities = nil
       s = Timeout.timeout(@open_timeout, Net::OpenTimeout) do
         tcp_socket(@address, @port)
       end
@@ -691,6 +692,7 @@ def do_start(helo_domain, user, secret, authtype)
         # authentication failed, cancel connection.
         s.close if s
         @socket = nil
+        @capabilities = nil
       end
     end