In other words, the encryption takes place between individual SMTP relays, not between the sender and the recipient. This has both good and bad consequences. A key positive trait of transport layer encryption is that users do not need to do or change anything; the encryption automatically occurs when they send email.
In essence, encryption occurs between intermediary mail servers, rather than directly between the sender and recipient. This has both advantages and disadvantages. A key benefit of transport layer encryption is that users don't need to do anything or change their behavior; the encryption happens automatically whenever they send an email.