[imp] Read receipts

[Chuck Hagenbuch]

Quoting Henry Blackman <h.blackman@chester.ac.uk>:

> X-Confirm-Reading-To: email@domain.com
> X-pmrqc: 1
> Return-receipt-to: email@domain.com

I don't see the harm, so I've added these in HEAD.

> Also, and more importantly, is IMP supposed to send a read receipt itself
> when a mail is opened that is flagged for the sender to receive a read
> receipt?  I'm wondering because that is probably our most requested features
> here... A "feature" I personally find annoying, but still.

No, it isn't.

-chuck

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