public final class MailSender

Sends mail through the server’s transport: what an application injects, as it would Spring’s JavaMailSender.

@Service
public class Signup {
    private final MailSender mail;
    public Signup(MailSender mail) { this.mail = mail; }

    @Transactional
    public void register(Account account) throws IOException {
        accounts.save(account);
        mail.send(Mail.to(account.getEmail())
                .subject("Welcome")
                .template("mail/welcome", new Model().addAttribute("name", account.getName()))
                .build());
    }
}

send returns at once. The mail is encoded on the calling thread and handed to the server’s mail thread, which delivers it, tries again when the mail server is unreachable and gives up when it refuses. Inside a database transaction the mail is written to an outbox table in that transaction and sent after the commit, so a rollback sends nothing and a crash between the commit and the send sends it when the server is back.

Where it goes is cn1.mail.url; see Mailers. Without one, mail is kept in memory and sent answers it, which is how a test reads what its code sent.