Another widely used "legal lifehack" of this kind is to just add "Attorney Privileged and Confidential" or something similar either to the first line of the email or to the subject and then (optionally) cc a member of legal team, thinking this will prevent the document being found and used in discovery. This won't work.
Attorney client privilege covers you providing facts to your legal team sufficient for them to give you some advice and them providing that advice.[2] You can't just tag any communication and think it is magically covered, and not all communications involving a lawyer are covered either. In an actual discovery (at least in the ones I have been involved in) there is a massive document production (including emails but every other kind of document also) and then the lawyers go into a conference to argue about what is and is not privileged. If you mark actually privileged conversations you can speed this process up because they don't need to sift through as much, but if you have marked a bunch of comms privileged that is not, all you are doing is making this process take longer (and thereby increasing your legal bill). Either way, you marking something (or not) doesn't actually affect whether it is privileged (or not).
[1] I can say from experience that "Mr Hunter, what do you think it means when it says here this was 'a complete clusterfuck'?" is not a fun question to have to answer to a roomful of lawyers but whatever.
[2] https://uk.practicallaw.thomsonreuters.com/7-506-8557?contex...