I can't tell yet if I'll actually end up using this in practice, but it's pretty well implemented.
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.
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.
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.
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.
Seems pretty well thought-out.