Many thanks to all for your replies. It looks as if the problem has stopped. It was propably due to a problem on the server.
There is a possibility that, for some reason, your email client cannot delete the message from the server - as a result you would get it each time you retrieve the mail. This may happen in very rare cases if the message somehow gets corrupted.
The solution in this case is to get to the server using a webmail feature, and delete the message from there. Or manage to retrieve your mail from another client for once. For instance from http://proz.mail.everyone.net
you can retrieve your mail from another server (go to "Options" / "Check external mail" /click on "Edit" and uncheck "leave mail on server")
If this is not the case, Ralf's advice is the only way to go.
[Edited at 2005-09-26 16:46]
[Edited at 2005-09-26 16:47]