I don't know IMAP beyond the very basics, but I presumed that it doesn't include facilities for adding metadata. Correct me if I'm wrong! So if you wanted to store snoozing info, you'd have to encode that oddly somewhere -- in a fake email item name? Or as an added mail header (if it's considered polite for clients to add them)?

Using a header works fine - making up your own is legal. I've been known to add headers for entertainment purposes.

You can't write headers to messages you receive.The only way is to download the message (including the three 78MB PDF attachments you got from that graphic designer), add the header field, upload everything and delete the first message.

But if your longish snoozes end at the same time, you can use a flag.

(IMAP's cache semantics say: If you download a part of the message it stays valid. Other people cannot change what you have downloaded.)

I'm aware of that. Not saying it is the right method, just that it is the best general method we have.

And I'm also thankful that I trade mail with far fewer designers these days.

As I remember IMAP allows to add freeform flags to every message.

IIRC that's how gmail implements its labels.

I don't think so, since gmail had labels before they added IMAP support. Also, when accessing gmail via IMAP, you see the labels as IMAP folders, which leads me to believe that they just bolted the IMAP interface on top of whatever backend they use to store messages internally.

I meant that that's how they implemented labels for their IMAP support, but you may be right, they may be folders instead.

I have used the subject for metadata as a way of storing backup files in IMAP. You can also use tags and as someone else mentioned you can do all sorts of things with headers.