[exim] sender verify and greylisting

Hello,

our mailserver refused a certain email of a customer and when we tried
to track down the problem, we noticed that the server of the server
replied to our verification like this:

mail from: <>
250 Ok
rcpt to: test@???
450 <test@???>: Recipient address rejected: Greylisted for 300
seconds (see http://...)

Might this be the reason for the sender verify to fail? If yes, what can
we do against it?

Regards
Marten