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You know how long it takes gmail servers to respond to an invalid recipient with a 550? About two seconds.

2 seconds? Not true at all, it's just tens of milliseconds. Didn't time it exactly, but it seems 'instant', so it's less than 100 ms.

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.