If I search for e.g. "filmas" in all lists on http://lists.debian.org/search.html
the result contains about 90% already removed spam messages.

The results 2,3 and 4 for example are:
http://lists.debian.org/debian-68k/2011/01/msg00036.html
http://lists.debian.org/debian-alpha/2011/01/msg00006.html
http://lists.debian.org/debian-amd64/2011/01/msg00013.html

Is the search index outdated or are removed spam messages
recognised as normal messages?

(Note: I reported this about half a year ago but cannot find
the report anymore.)

Thanks for reporting this, and sorry that it's not been addressed
for so long.

The main issue is that we don't currently have a process to purge
removed spam from the index.  I have been working on that, but it's
not ready to deploy yet.

However, we've just rebuilt the search index from scratch after
upgrading the server to stretch, which means there's now nearly
one million fewer already removed spam messages indexed - I just
checked your example search and it currently returns valid links
(many are spam, but spam that exists in the archives).

Cheers,
    Olly

Final Notice.

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Thank You.

Regards
Mr. Rowland Cole
( Financial Crimes Enforcement Network)