"PA Bear [MS MVP]" ha scritto nel messaggio
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Does the behavior persist when you access the same newsgroup via the
same
newsserver in a new identity?

now I'm trying with a new identity and apparently the problem does not
arise. Do you think my first identity has been marked as "bad" by my
provider?

No, I think your original identity was/is damaged, especially if it is the
default Main Identity.

Thank you very much for your valuable advices, I'll follow them soon
anyway I can't understand how the items you quoted can damage my identity,
for instance why folder compacting is recommended, can you suggest some
explanatory link?
il barbi