[fetchmail]fetchmail: Query status=24

I am running fetchmail release 6.2.5+POP2+RPA+NTLM+SDPS+SSL+OPIE+NLS on 
suse 9.1 with unlimited 512 broadband. Delivery is IMAP through postfix. 
Every now and again someone sends me a large email which causes this 
diagnostic:
fetchmail: Query status=24

And further mail can't be retrieved until I delete the message manually 
using the ISPs web interface. Server timeout is 300s.
Just wondering how to diagnose this. Is it likely to be a fetch mail 
feature, or something on the ISP server?
Berni