A mailing list not being recognized as such

Dear all,

I notice that mutt does not identify a mailing list to which I am
subscribed as expected.

In Mutt's configuration, I used the content of the List-Post header as
parameter of the subscribe command.

However, mutt does not display the 'L' flag I would like to see for
messages coming from that list in its index. At the same time, if I try
to respond to the list this works and the address to which the mail is
sent is correct, namely the one in the List-Post header.

I observe that some messages are sent to the list in Bcc, but I am not
convinced this is relevant since I don't have the 'L' flag for any
message sent to the list.

Any idea?

Thanks,

Seb.