I can not fetch emails

Hi, all.

If I turn on the Spam-Filter in my Endian-Firewall and press Fetch Mails, the text "fetching mails..." is displayed until I press another button.
And there are no new mails.
But if I turn off the Spam-Filter, I can fetch all mails.
I�m sure, that my testmails are not spam, because it works with other email-clients behind the same firewall.

Can anyone help my?
And sorry about my english, but I think you understand what I mean.

Thank You,

Oliver