This seems to be a concert for many Mastodon newcomers so I will address it in this toot:
As the instance admin, I have the ability to read your DMs. They're unencrypted.
There are two ways I can do this:
1- Looking directly into the instance's database
2- If someone reports a DM you send
I will never do the former. However, the latter is a different story, as I need to moderate them to keep the instance and its users safe, and unless not doing so threatens that safety, I will respect your privacy.
If you need to exchange sensitive information that required absolute secrecy, I'm strongly recommending you to use any encrypted method of communication isntead of Mastodon DMs.
Thank you for your attention and understanding!