[fetchmail] forcing the retrieve option

hi 

i am having trouble with a mail sever of my service provider. he is using 
microsoft exchange as the server. while fetching mails from this server 
using fetchmail 5.9.11 i am getting the message "message delimiter found 
while scanning headers". also sometimes fetchmail exits with a message 
"Query status = 2" "Client server synchronisation failed". i am also 
fetching mails from some other servers also and they dont exhibit this 
behaviour. i have a 1 Mb Leased line to internet so there is no possibility 
of connectivity problems. if i use the "fetchall" keyword (thus forcing 
retrieve)with the accounts in this server it works without problems. problem 
is i want to use the keep option with a particular account in this server, 
but when i use fetchall it will retrieve the already retrieved mails again 
which i dont want it to. is there any other way to force retrieve option 
other than the "fetchall" keyword ? 

i am a new to this list. i did search the archives for any info on this. but 
apologies in case this has been already answered in this list. 

best regards 

bobby