I just realized the following.
The domain meccanotecnica.it apparently has no MX record. (See https://tools.dnsstuff.com/#dnsReport|type=domain&&value=meccanotecnica.it)
That is perfectly legal, and mailer daemons in such a case are supposed to try to send to the A record of the domain (which in case of this domain *does* indeed accept mail for that domain).
GWIA 18.2.1 however attempts to connect to 127.0.01 in that case instead of the A record, which in most cases produces a 4XX error and useless retries (as on most servers, postfic will be running on 127.0.0.1, reacting with a relaying denied, and when postfix does not run, the result will be a host down error, again delaying the mail).