Plus addressing not working

[Diego Augusto Molina]

Hi, I have setup a server with Postfix + Dovecot supporting plus 
addressing. I can verify that by sending a mail to a specific folder 
like 'user+spam at domain.com'. I also have amavisd-new 2.6.5 in another 
server, which does it's job pretty well except for not sending to the 
spam folder.
I can see in the logs something like this:

Jun  2 11:41:52 server amavis[4994]: (04994-13) Passed SPAMMY, 
[176.31.244.10] [176.31.244.10] <info at mardatas.com> -> 
<user at domain.com>, Message-ID: 
<40f2f25a764dc627e320c1489c864503 at mardatas.com>, mail_id: QvhJRnzFVAji, 
Hits: 8.263, size: 11551, queued_as: 46FEF462276F, 
dkim_id=@mardatas.com, 5858 ms
Jun  2 11:42:54 server amavis[4992]: (04992-18) Passed SPAMMY, 
[66.196.80.165] [66.196.81.181] 
<sentto-76244287-2141-1464878618-user=domain.com at returns.groups.yahoo.com> 
-> <user at domain.com>, Message-ID: 
<E0964223-50E3-4DDF-A77F-551B2B6D903F at ymail.com>, mail_id: eG7MRvM3DlKe, 
Hits: 6.785, size: 20434, queued_as: C26BA462276F, 
dkim_id=@yahoogroups.com, 6017 ms

I have set up the following in "/etc/amavis/conf.d/50-user":

$recipient_delimiter = '+';
@addr_extension_spam_maps = ('spam');

Am I missing something?