For patent attorneys and agents who you have engaged, any disclosures you make to them are confidential and privileged. They would not be able to make any of the matter public (at the risk of losing their licence to practice). You should therefore feel quite comfortable disclosing matter to your attorney/agent.
However, they would likely want to have accepted you as a client beforehand before you send a patent application, to ensure they have no conflicts. And that's just politeness really.
Anything you write in the body of the email is more or less irrelevant to whether the content is confidential or not. If you have disclosed your invention, it's been disclosed, regardless of whether you say you intended to or not (since by sending the email, you clearly intended to disclose it).
In some cases (and specifically for large US corporates where litigation is a real likelihood), it may be prudent to explicitly mark communications with your attorneys as being privileged, so as to facilitate the inevitable discovery process. But that's not going to really change whether it's privileged or not, it just makes it clear.
Similarly, where the recipient has a duty of confidence (such as after they have disclosed an NDA), it may be useful to mark communications as being confidential, as a prompt to remind them of their duty. But you can't unilaterally import a duty of confidence on someone just by saying "this is confidential" at the top of an email.
As an aside, it's for this reason that those ridiculous paragraphs at the end of emails about unintended recipients make no sense. They have no legal force, and just waste everyone's time.