I can't tell yet if I'll actually end up using this in practice, but it's pretty well implemented.<p>The memo is stored as another email in the "Memos" folder as a reply to the message being memoed, so the data will be backed up naturally by any email backup and does show up, in a sort-of-awkward way, in other non-Fastmail mail clients.<p>In the Fastmail client, the memo shows up, highlighted, in the message list before the normal message snippet, and at the top of the message when you edit the message.<p>Seems pretty well thought-out.
I love this, as it's not something I'd have ever really considered as a useful feature, but now that I see it I'd like to find ways to use it