Everything that comes in to the mail server has to pass by the greylisting feature, so if you haven't sent an e-mail to me, kimh at os2world dot com, the MTA (Postfix in our case) will say that it's for the moment a little bit busy and that what ever mailserver trying to connect has to try again later. So far so good, so when you once has been verified as trusted sender, your added to the greylist for 90 days. If you don't during this period communicate with the server, your e-mail is flushed out of the greylist.
Weasel shouldn't have any issue with this, go ahead and give it a spin and see what turns up in the log files from Weasel. I don't know if Weasel has greylisting feature but it might be a great suggestion for future enhancement (or bounty suggestion) since spam has increased. Read somewhere that spam takes up around 85-90% of all e-mails sent... bandwidth that someone has to pay for.
So far the only e-mails that we had to put on the whitelist was from Google for some of the various services that they have and that we use.