Lots of services already depend on email verification during signup or password recovery. I'd love to know, have you seen any interesting solutions around greylisting in the wild?
To "bypass" or somehow defeat greylisting, you mean?
I'm not aware of any via workarounds, no. I would certainly be interested in hearing about any , though, since I use greylisting on mail servers with thousands of users.
In some cases, such as when using OpenBSD's spamd , you (an SMTP client) don't even get to talk to the "real" SMTP server until you've successfully "passed the tests". This is the primary implementation of greylisting that I use, FWIW.