[Dovecot] Performance problem with outlook & outlook express

Hi, I've set up a dovecot server under redhat enterprise 4.3 with the basic configuration(mbox). Everything runs fine under webmail(squirrelmail) and thunderbird. When I try it under outlook/outlook express tough, it takes a couple of minutes after the authentication to get all headers where it takes a couple of seconds under thunderbird(both windows and linux version).

I tried with these two versions with same results: dovecot-1.0.beta9 and dovecot-1.0.rc7

Any idea? Thanks for help and regards