[Symfony4] Swift_TransportException: Expected response code 250 but got an empty response.

I made custom console command  for sending email letters (using spool). When I manually run command, and when I use your service (Command Scheduler) immediately after the command **php bin/console cache:clear**, then all working, but when the day goes then this service returns a Swift_TransportException "Expected response code 250 but got an empty response". Why is this happening?