Exchange + ISP email make Outlook hang

We are using an Exchange 2000 server but we have our internet email hosted at our ISP. Everything has worked fine for several years until last monday. Without any apparent reason, Outlook now will lock up when trying to send internet mail. This only happens when "Internet mail" is at the top of the "Delivery" list in "Settings". If we send out via the exchange server, everything works OK. Internal emails work fine as well (I mean, when using Exchange addresses instead of internet addresses). I have seen the exact same problem described in an other discussion forum but the guys didn't get any proper answer. We have of course dowloaded all the known updates and patches available from Microsoft but this didn't help.
Could anybody sort us out? Thanks in advance.
Roger