Linux – sendmail is not delivering messages to recipients

TLS just enables encryption on the smtp session and doesn't directly affect whether or not Postfix will be allowed to relay a message.

The relaying denied message occurs because the smtpd_recipient_restrictions rules was not matched. One of those conditions must be fulfilled to allow the message to go through:

smtpd_recipient_restrictions =
    permit_sasl_authenticated
    check_recipient_access hash:/etc/postfix/filtered_domains
    permit_mynetworks
    reject_unauth_destination

To explain those rules:

permit_sasl_authenticated

permits authenticated senders through SASL. This will be necessary to authenticate users outside of your network which are normally blocked.

check_recipient_access

This will cause postfix to look in /etc/postfix/filtered_domains for rules based on the recipient address. (Judging by the file name on the file name, it is probably just blocking specific domains... Check to see if gmail.com is listed in there?)

permit_mynetworks

This will permit hosts by IP address that match IP ranges specified in $mynetworks. In the main.cf you posted, $mynetworks was set to 127.0.0.1, so it will only relay emails generated by the server itself.

Based on that configuration, your mail client will need to use SMTP Authentication before being allowed to relay messages. I'm not sure what database SASL is using. That is specified in /usr/lib/sasl2/smtpd.conf Presumably it also uses the same database as your virtual mailboxes, so you should be able enable SMTP authentication in your mail client and be all set.

"helo command rejected" means that your "A" server didn't even get as far as the first verb in the SMTP Relay conversation. The envelope sender and recipient are thus not the problem. They haven't even been sent at the point that the error occurs.

The error is response code 554, extended response code 5.7.1. This is the conventional "I don't want to talk to someone at your IP address." response code. The reasons why are up to the server owner, and can vary from

This blacklist says that you are a third-class Internet citizen and I am treating you as such.

to

There isn't actually an SMTP Relay server here, you know.

In this particular case, the reason appears to be

The name localhost.localdomain isn't uniquely yours, and I've decided not to talk to people who configure their MTSes with that as the supposedly unique name of their MTS. You know where the confDOMAIN_NAME setting in Sendmail is.