Error trying to send email with Symfony Mailer

Question

How do you configure Symfony Mailer? I'm swapping Swiftmailer to Symfony Mailer because Swiftmailer is now deprecated. However, I haven't been able to send a single email with Symfony Mailer yet. Swiftmailer always works and the emails are caught by smtp4dev.

Platform:

• Windows 10
• Symfony 4.4
• smtp4dev listening on port 25

mailer.yaml

framework:
  mailer:
    dsn: 'smtp://localhost'

Error message:

stream_socket_enable_crypto(): SSL: An established connection was aborted by the software in your host machine.