DSL:MailSendingMessageHandler

Actually I don't see reason of this Builder. It can be absorbed just with `MailSendingMessageHandlerSpec`.
Right, I understand the case, when end-user can provide his own `JavaMailSender` impl, but can achieve that with separete `public static MailSendingMessageHandlerSpec outboundAdapter() {`.
You already provide one for the external `JavaMailSender` and there is need to have a new one to absorbe this builder in the `MailSendingMessageHandlerSpec`.
When `outboundAdapter(JavaMailSender sender) ` can return just `MailSendingMessageHandler` instance, because we don't have any other options to configure.