Well, thank you for your kind help!
I`m using Windows XP Professional.
Browser: Firefox.
As you pointed out, it might be the browser.
I tried to upload an attachment with the three mentioned providers running with Firefox 1.5.0.1 and then with IE 6.0
Firefox didn`t work.
IE managed to upload a largeish attachment, when Standard resident protection was deactivated.
Otherwise, that is while the other three providers were running, Google Mail brought up an error message saying "Ops, couldn`t do that" or something similar.
And there is no spectacular information on the mail scanner log, as far as I can judge that. Only five or six lines.
Shall I send you a Google Mail invitation, so you may can reproduce that error?