Yes, OxiBounce is one of the few solutions (perhaps the only one?) that tests addresses protected by greylists. For those unfamiliar with the concept, a greylist is an anti-spam measure that temporarily rejects messages with a temporary error. After a certain time, the mail server accepts the message.

For performance reasons, we limit the wait time to a few minutes. If the greylist threshold is not reached within this time, the address will be marked as “NOTSURE” with the “ERR_GREYLIST” error code.

No need to worry about these addresses, though: they are valid addresses, and this protective measure actually reinforces the idea that the email server is well-maintained.