Re: [client] did not issue MAIL/EXPN/VRFY/ETRN during connection

Claus Aßmann wrote:

On a positive note, I got information from the remote end:

The site which tries to connect to your sendmail server?

Correct.

Remote server returned "554 5.4.0 < #4.4.2>"

"Remote server" would be you MTA -- but that doesn't look like
anything sendmail would return -- unless you have "weird" entries
in the access map.

Zero entries related to these destinations. Mostly entries to allow relaying from internal networks, and options to disable, such as below. Internal entries and exceptions removed - but once again, none for any of the problematic sender domains.
# more /etc/mail/access
# Internal hosts whitelisted off DNS BL checks
Connect:127.0.0.1                       RELAY
# Trusted subnets and domains
127.0.0.1                               RELAY
domain.com                             RELAY
# Exceptions for broken systems
# Refer to section "Allowing Connections" on Sendmail's cf/README file
#
# Srv_features:
# RFCs require CR LF . CR LF for end of email message. Many systems won't respect that. Set
# G to allow for end of email with single LF
# U to allow for end of email with single CR
# tls_server: called when sendmail act as client, ready to send an email
# message after a STARTTLS command (should) have been issued;
# tls_client: called when sendmail act as server, ready to receive an email
# message after a STARTTLS command (should) have been issued, and/or from
# check_mail;
# Default TLS settings
Try_TLS:                                YES,VERIFY,ENCR
TLS_Srv:                                VERIFY,ENCR
TLS_Clt:                                VERIFY,ENCR