Well, Google never can enable pre-caching of images - simple reason: it would allow an instant check if the email address is valid. Just send a mail and wait a bit - and you'd know if the mail was valid.
Maybe it hiccuped? It seemed instant for a valid address, but a little slower for invalid. I assumed it keeps used addresses in cache but had to go digging to confirm a negative, but it could be a fluke. Anyway, it's faster than waiting for the image proxy.
> it would allow an instant check if the email address is valid.
What's bad about that?
Not to mention you can do it anyway - just try to send an email to an address and see if they accept it, if they do then send the server a reset command so it discards the email.
Not if Google also requested images from invalid addresses. But usually you'll get a failure in the SMTP transaction if the email address is not valid. Otherwise a simple typo would send your important email into a black hole.
