Postfix white listing a badly configured server not working: Helo command rejected: Host not found;

My mail server configuration is rather strict and some times incoming mails from legit servers get rejected because the remote end has a configuration problem, a common one is the HELO hostname.

I have a specific example that is from the most important electrical company in Italy, ENEL, that apparently sends transactional emails with an HELO hostname that does not resolve:

Feb 20 18:31:10 MYHOST postfix/smtpd[1748649]: NOQUEUE: reject: RCPT from mxrelay6.enel.com[146.133.127.102]: 450 4.7.1 <smtprelay.enel.com>: Helo command rejected: Host not found; from=<[email protected]> to=<MY CLIENT EMAIL> proto=ESMTP helo=<smtprelay.enel.com>

I am trying to whitelist this, the IPs or hostnames of the smtp server seems the logical step (I have observed two but I could use a regexp). Anyway, I am currently unable to, anything I try does not work and those emails always get rejected. (I have other similar cases).

##
# Helo restrictions
smtpd_delay_reject = yes
smtpd_helo_required = yes
smtpd_helo_restrictions =
  permit_mynetworks,
  warn_if_reject check_helo_access hash:/etc/postfix/helo_whitelist,
  warn_if_reject reject_invalid_hostname,
  warn_if_reject reject_unknown_hostname,
  warn_if_reject reject_non_fqdn_helo_hostname,
  warn_if_reject reject_unknown_helo_hostname,
  warn_if_reject reject_invalid_helo_hostname,
  permit

##
# Sender restrictions
smtpd_sender_restrictions =
  permit_mynetworks,
  check_client_access hash:/etc/postfix/sender_whitelist,
  reject_non_fqdn_sender,
  reject_unknown_sender_domain,
  reject_unknown_address,
  reject_unknown_reverse_client_hostname,
  reject_unknown_client_hostname,
  permit

##
# Recipient restrictions
smtpd_recipient_restrictions =
   permit_mynetworks,
   reject_unauth_pipelining,
   reject_unauth_destination,
   reject_invalid_hostname,
   reject_unknown_hostname,
   reject_unknown_reverse_client_hostname,
   reject_unknown_sender_domain,
   reject_unknown_recipient_domain,
   reject_non_fqdn_hostname,
   reject_non_fqdn_sender,
   reject_non_fqdn_recipient,
   check_sender_access hash:/etc/postfix/sender_access,
   #check_client_access hash:/etc/postfix/rbl_whitelist,
   check_policy_service unix:private/policyd-spf,
   #check_client_access hash:/etc/postfix/rbl_override
   reject_rhsbl_helo dbl.spamhaus.org,
   reject_rhsbl_reverse_client dbl.spamhaus.org,
   reject_rhsbl_sender dbl.spamhaus.org,
   reject_rbl_client zen.spamhaus.org,
   #reject_rbl_client cbl.abuseat.org
   #reject_rbl_client b.barracudacentral.org,
   #reject_rbl_client bl.spamcop.net,
   #reject_rbl_client zen.spamhaus.org=127.0.0.[2..11],
   #reject_rbl_client b.barracudacentral.org=127.0.0.[2..11],
   #check_policy_service unix:/var/spool/postfix/postgrey/socket,
   permit

smtpd_relay_restrictions =
   permit_mynetworks, 
   permit_sasl_authenticated,
   reject_unauth_destination

##
# Data restrictions
smtpd_data_restrictions =
  reject_unauth_pipelining,
  permit

helo_whitelist

mxrelay7.enel.com OK
mxrelay6.enel.com OK

I have tried putting in helo_whitelist the actual HELO hostname smtprelay.enel.com and it doesn't work. I have seen in the docs also the use of PERMIT instead of OK but going through the debug messages (I used debug_peer_list) it looks like it is not correct.

Looking through the debugging though I can see that the reject happens in the recipient address restrictions:

Feb 20 18:41:12 alice postfix/smtpd[1749156]: reject_invalid_hostname: smtprelay.enel.com
Feb 20 18:41:12 alice postfix/smtpd[1749156]: generic_checks: name=reject_invalid_hostname status=0
Feb 20 18:41:12 alice postfix/smtpd[1749156]: generic_checks: name=reject_unknown_hostname
Feb 20 18:41:12 alice postfix/smtpd[1749156]: reject_unknown_hostname: smtprelay.enel.com
Feb 20 18:41:12 alice postfix/smtpd[1749156]: lookup smtprelay.enel.com type A flags
Feb 20 18:41:12 alice postfix/smtpd[1749156]: dns_query: smtprelay.enel.com (A): Host not found
Feb 20 18:41:12 alice postfix/smtpd[1749156]: lookup smtprelay.enel.com type AAAA flags
Feb 20 18:41:12 alice postfix/smtpd[1749156]: dns_query: smtprelay.enel.com (AAAA): Host not found
Feb 20 18:41:12 alice postfix/smtpd[1749156]: lookup smtprelay.enel.com type MX flags
Feb 20 18:41:12 alice postfix/smtpd[1749156]: dns_query: smtprelay.enel.com (MX): Host not found
Feb 20 18:41:12 MYHOST postfix/smtpd[1749156]: NOQUEUE: reject: RCPT from mxrelay6.enel.com[146.133.127.102]: 450 4.7.1 <smtprelay.enel.com>: Helo command rejected: Host not found; from=<[email protected]> to=<MY CLIENT EMAIL> proto=ESMTP helo=<smtprelay.enel.com>
Feb 20 18:41:12 alice postfix/smtpd[1749156]: generic_checks: name=reject_unknown_hostname status=2
Feb 20 18:41:12 alice postfix/smtpd[1749156]: >>> END Recipient address RESTRICTIONS <<<

Question is why the smtpd_recipient_restrictions section should check the helo hostname? Or is it just a delayed check because of smtpd_delay_reject?

Basically, where and how should I put the whitelisted helo messages?

Thanks!