451 Could not complete sender verify callout

[Phil Breskey <pbreskey@dotcomholdingsofbuffalo.com>]

Hi, I am having trouble mailing a few domains out there on the web. 
When we try to send them an email, it will stay in the queue for a few 
days, then come back to us with the error "451 Could not complete sender 
verify callout".  I've checked on Google, but didn't really find 
anything pertaining to qmail (save a few Exim people bitching about it). 
 The archive for this list (http://www-archive.ornl.gov:8000/) is 
timing out, and the other two listed on Life With qmail were not any 
help.  Can anyone give me a kick in the right direction?  Thank you.
Phil