Re: By default...
You might be able to ensure you have SSL between you and your mail server, but then as far as the protocol is concerned, that SMTP server could be delivering the message to the next relay by semaphore
Technically true. However, many organisations require their partners, vendors, etc to prove that TLS is in use and enforced, or at least available opportunistically at each hop, mail system to getway, gateway to filtering service provider, and vice versa