Command: addrcheck

  Format: addrcheck <level>

Sets the address verification level for the currently selected list.
At levels above zero, listmanager will attempt to validate the address
of a potential subscriber or submitter to your lists before permitting
a given operation to proceed.  This includes addresses that would normally
be added to your list as a result of the presence of the "auto-subscribe"
flag.

NOTE: If "auto-subscribe" is set on a list and the address check level
is not 0, and a submission arrives from an address for which the address
check fails, "auto-subscribe" is ignored for that submission.

The <level> defines how thorough listmanager's check is.  The following are
the valid check levels at present.  Note that checks are cumulative, so
(for example) the level 2 checks include all of the level 1 checks.

Level 0: No check

  No checking is done.  All addresses are considered valid.

Level 1: Syntax checking only

  The address is divided into the user part and the host part.
  The following must all be true for the test to pass:

    - the user part and the host part both exist (e.g. not just a
      userid; there must be an "@" or "%" in there someplace)
    - the host part does not end with a period
    - the host part contains only letters, numbers, hyphens and dots,
      with no adjacent dots and no terminating dot (as per RFC1034
      section 3.5)
    - the user part and the host part both contain 
    - the user part and the host part must not contain any non-printable
      characters
    - the user part must contain at least one alphanumeric character
    - the user part must not contain a comma

  This tests the syntax of the address, but does not test its validity.
  (e.g. A sentence can be properly structured, but it could also be a lie.)

Level 2: MX lookup

  The host part of the address is passed to the nameserver to see if
  any Internet-connected machine has been designated as accepting mail
  for that host.  This includes the RFC974 default MX, which means even
  if there are no nameserver records announcing "x accepts mail for this
  host", it will be assumed that "host" accepts mail for itself.  This
  generally verifies that the host part is valid, since someone out there
  wants to accept mail for that host or domain.  If there's no known way
  to get mail to the specified host, this test fails.

Level 3: SMTP "RCPT TO:" attempt

  The MX list obtained during level 2 testing is used to attempt to
  deliver a message to the specified recipient using SMTP.  If a remote
  SMTP server is reachable and rejects the specified address with a
  550 SMTP message ("User unknown"), this test fails.

NOTE: The level 2 and 3 checks are *NOT* conclusive.  Many mailers out
there blindly accept mail for any address during SMTP, but then reject
it later.  There's no way for listmanager or any other SMTP client
to deal with this case.  This feature is added only to make a good attempt
to weed out more bogus addresses; it can't possibly pick off all of them.

NOTE: This feature replaces the "check-addrs" flag.  Lists with the old
flag set will have this value automatically set to level 1 unless
it has already otherwise been assigned.

See also: auto-subscribe select