found some warning on maillog

[Mark Martinec]

>> If an undefined value was really intended, it should be represented
>> in LDAP (or in SQL) as a NULL, not some string.

> Thank you very much for your message and guide,
> I already checked and see some users have "amavisSpamTagLevel = undef"
> in ldap data.
> 
> If I input some value into this config, the warning will fix. Right ?

Right.

Either put a numerical value there, or if you'd like for a LDAP
lookup on such attribute to fall back to a static default,
put a NULL attribute value there.

  Mark