[fetchmail]"Query status=15" errors, what causes them?

I'm running fetchmail through an ssh pipe using 'preconnect', I'm
quite frequently getting "Query status=15" errors when fetching mail.
What causes these errors and is there anything I can do about them?

The mail still gets delivered which is a bit of a nuisance as it's not
cleared off the server when the erro occurs so I get it twice (or
oaccasionally even more times).

-- 
Chris Green (chris(_at_)areti(_dot_)co(_dot_)uk)