It's not uncommon to get an email with a vague message like "here's the info you requested" with an attachment like "slides.pptx". If I create a task from this email, I have the ability to add a note (although it appears task notes are currently not searchable) to remind me why the email is relevant. However, if I just need to hang onto this email for reference, I don't see a way to annotate it short of forwarding it to myself with my notes as the content of the forwarded message. I would find it very useful to be able to attach notes to any message and have the contents of the notes be searchable. This is somewhat complementary to the separate idea to be able to add tags to emails in that keywords could be added in the notes to achieve a similar effect. In the long term, I think both tags and notes would be useful, but searchable notes would likely be much easier to implement in the short term and could be used as an interim solution for those wanting the ability to use tags.