@@ -183,6 +183,15 @@ func (s *SMTPHandler) dispatch(subject, htmlBody, plainBody, to string) Delivery
183183if err != nil {
184184return true ,xerrors .Errorf ("message transmission: %w" ,err )
185185}
186+ closeOnce := sync .OnceValue (func ()error {
187+ return message .Close ()
188+ })
189+ // Close the message when this method exits in order to not leak resources. Even though we're calling this explicitly
190+ // further down, the method may exit before then.
191+ defer func () {
192+ // If we try close an already-closed writer, it'll send a subsequent request to the server which is invalid.
193+ _ = closeOnce ()
194+ }()
186195
187196// Create message headers.
188197msg := & bytes.Buffer {}
@@ -250,7 +259,7 @@ func (s *SMTPHandler) dispatch(subject, htmlBody, plainBody, to string) Delivery
250259return false ,xerrors .Errorf ("write body buffer: %w" ,err )
251260}
252261
253- if err = message . Close ();err != nil {
262+ if err = closeOnce ();err != nil {
254263return true ,xerrors .Errorf ("delivery failure: %w" ,err )
255264}
256265