[Zope] Mailing list archives not readable with mutt ?

For some reason, you can't read the downloadable versions of the mailing list archives with mutt--it fails to split the mbox into messages. Strangely, mailx just works fine. I tried to use procmail's formail to split and concatenate the messages, but somehow that fails, too. Has anybody a hint how I can make the mbox suitable for mutt ? Gregor