Greylisting

[David Rosenstrauch]

I've been a member of this list for years.  But I recently had an email 
to the list rejected due to greylisting:

<xfce at xfce.org>: host mail.xfce.org[138.48.2.106] said: 450 4.7.1
     <xfce at xfce.org>: Recipient address rejected: Greylisted for 5 
minutes (in
     reply to RCPT TO command)

Any idea what's going on here?  I thought that greylisting simply 
delayed emails, not rejected them.  (BTW, my email provider is 
Dreamhost.)

Thanks,

DR