Please do not use #Mastodon to communicate privately or think it's secure. Use Signal, Wire.com, XMPP, Matrix.org, Tox.chat, or your preferred choice.
Toots are not stored encrypted & you also may not know the server's policy where you're @'ing someone if it's not your home Mastodon server/instance & can't confirm deletion.
I only use DM to not flood timelines & to keep a public conversation between A, B, C...
Toot safely.