Interactive DNS MX-verify CGI running at blackhole.arlott.org.uk

Enter the domain, or email address for which you want to have the backup delivery things verified.

The domain name (or address) you give below must have MX records for it. If there are no MXes for given domain, email might get delivered by means of using the traditional ``if there are no MXes, use A'' operational rule in the internet. However this utility won't (naturally) have any useful work in that case, and it will report an error (but continue).

This does not send any email to you, it just does a series of connections, plus interactive destination address tests of following kind:

  MAIL FROM:<>
  RCPT TO:<postmaster@that.domain>
  RCPT TO:<abuse@that.domain>
  RCPT TO:<your.address@that.domain>

The explanation:

• The ``MAIL FROM:<>'' is the RFC 821 defined error message transport, which many systems fail to allow -- they must not do so.
• Every email receiving domain must have postmaster account, according to RFC 2142.
• Every email receiving domain must have abuse account, according RFC 2142.
• Finally (if you have given an address, and not just a domain), also tries to use the address you gave.

For each server, it is acceptable to return:

• ``ERROR: Connection Timed Out'' -- server wasn't reachable within tester's short 20 second timeout period; this may be due to firewall blocking access to the primary servers with the intention that incoming email comes through backup servers only, or broken TCP ECN handling. (More usual connection timeout is 5-10 minutes.)
• ``ERROR: Connection Refused'' -- either the destination server does not have an SMTP server running, or a network firewall near the destination server blocks access to it with the intention that email comes through backup servers only, or broken TCP ECN handling.
• ``OK'' -- the address tests gave an OK result.

Anything else means you have to figure out what is going on. Especially if any backup server rejects the ``RCPT TO'' with 400- or 500-series codes.

Domain or Email Address:

This server supports both IPv4 and IPv6 and has TCP ECN enabled.

All tests this utility does must result in ``OK'' if you aspire to have any chance at having reliably working incoming email. (or ``Connection Timeout'' or ``Connection Refused'')

This CGI utility is one of ZMailer auxiliary utilities, and it was written by Matti Aarnio after he got sufficiently annoyed about failing backup-MX servers for lots of different domains gave reports of type:

• ``not relaying to that domain''
• ``you are not allowed to relay to that domain''
• ``this is not our relaying customer''
• ``sorry, that domain isn't in my list of allowed rcpthosts''
• ``This target address is not our MX service client.''

the most common of which are rather misleading wordings about what is going on; that the server administrator has not listed allowed relay destinations into some file...