out.alice.it Delivery Failed and MX records

I was getting messages like the following form the MTA of my ISP(Alice/Telecom Italia) when sending email to some addresses:

Subject: Delivery Status Notification

 - These recipients of your message have been processed by the mail server:
openezx-devel@lists.openezx.org; Failed; 5.4.0 (other or undefined network or routing status)

Reporting-MTA: dns; smtp207.alice.it
Received-from-MTA: dns; jcn (80.116.xxx.xxx)
Arrival-Date: Thu, 17 Mar 2011 19:52:43 +0100

Final-Recipient: rfc822; openezx-devel@lists.openezx.org
Action: Failed
Status: 5.4.0 (other or undefined network or routing status)

Many other users of alice.it were experiencing the same problem with different addresses.

It turned out this was happening because the domain name of the final recipient did not have an MX record configured in the DNS setup, and some MTAs are not happy with that. However this behavior is overly strict according to the RFC5321: SMTP specification (or my interpretation of it), quoting from section 5.1. Locating the Target Host (emphasis is mine):

If an empty list of MXs is returned, the address is treated as if it was associated with an implicit MX RR, with a preference of 0, pointing to that host.

I couldn't find out what the exact definition of “implicit MX” is, can it be either an A or AAAA record?

I am still not sure if the Alice MTA is violating the specification here, or if the fallback to an A record is deprecated by some other RFC, anyone?

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