Bad sender address syntax ..

[Amedeo Rinaldo]

Weird 'Bad sender address syntax', where '""@' come from ? some ideas? 
(see info below)

-- 
  .''`. *Amedeo Rinaldo*
: :' : debian addicted ;-)
`. `'
   `-

-------------------


OS: Debian squeeze (2.6.32-5-amd64)
Amavis: debian 2.7.0-1~bpo60+1 (amavisd-new-2.7.0 '20110701')
Postfix: debian 2.9.1-2~bpo60+1

(real data masked)

## mailq

-Queue ID- --Size-- ----Arrival Time---- -Sender/Recipient-------
4082940175     4252 Wed Jul 11 09:16:46  ""@externaldomain.tld
(host 127.0.0.1[127.0.0.1] said: 451 4.5.0 Error in processing, 
id=01823-04, quar+notif FAILED: Can't quarantine: 501 5.1.7 from 
MTA(smtp:[127.0.0.1]:10025): 501 5.1.7 Bad sender address syntax at 
/usr/sbin/amavisd-new line 14443. (in reply to end of DATA command))
                                          recipient at mydomain.tld
-- 4 Kbytes in 1 Request.


## mail.log snip
..
Jul 11 09:16:46 mx20 postfwd2/policy[8975]: [RULES] rule=3, id=WL_003, 
client=smtp.externaldomain.tld[111.222.333.444], 
sender=<@externaldomain.tld>, recipient=<recipient at mydomain.tld>, 
helo=<smtp.externaldomain.tld>, proto=ESMTP, state=RCPT, delay=0.00s, 
hits=WL_003, action=DUNNO
Jul 11 09:16:46 mx20 postfix/qmgr[4250]: 4082940175: 
from=<@externaldomain.tld>, size=4252, nrcpt=1 (queue active)
Jul 11 09:16:47 mx20 re-injection/smtpd[11314]: warning: Illegal address 
syntax from localhost[127.0.0.1] in MAIL command: <""@externaldomain.tld>
Jul 11 09:16:47 mx20 amavis[11835]: (11835-17) (!)SEND from 
<""@externaldomain.tld> -> <quarantine at localsystem.tld>,BODY=7BIT 501 
5.1.7 from MTA(smtp:[127.0.0.1]:10025): 501 5.1.7 Bad sender address syntax
Jul 11 09:26:29 mx20 postfix/qmgr[4250]: 4082940175: 
from=<@externaldomain.tld>, size=4252, nrcpt=1 (queue active)
Jul 11 09:26:30 mx20 re-injection/smtpd[11722]: warning: Illegal address 
syntax from localhost[127.0.0.1] in MAIL command: <""@externaldomain.tld>
Jul 11 09:26:30 mx20 amavis[12339]: (12339-14) (!)SEND from 
<""@externaldomain.tld> -> <quarantine at localsystem.tld>,BODY=7BIT 501 
5.1.7 from MTA(smtp:[127.0.0.1]:10025): 501 5.1.7 Bad sender address syntax