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Nice. To be clear, if I signed in with my memo key and started this message with a '#', you would be the only one that could see it as it is a reply to your message?

No. It just works with an Memo in a transfer. I thought this is what @xawi means.
The content of the memo is encrypted but everyone can see who it was send to.

Thank you for the clarification. So if I logged in with the memo key, sent you .001SBD and wrote '# hi there security101' in the memo field, nobody would see what I typed as it would be encrypted. I just want to be clear for the other users so that they know how it works.

Yes that’s right !
Of course when doing a transaction from the wallet you need to provide the private active key.