I use EFA spam filter server in front of a Slackware server with Sendmail. The spam filter forwards the messages to the Sendmail server. The users are known on the Sendmail server and can send email through this SMTP server (with TLS and authenticated) by using port 25 from everywhere (mobile phones). So far so good.
The issue is with Sendmail that accepts mail from every host to deliver locally. I actually want Sendmail to be available for sending emails from local known users, but not delivering emails to local users other than from the spam filter server.
Although this is not being used by spammers for now (the MX record is the EFA spam filter) it is possible to send mail to local recipients without going through the spam filter if i set the receiving SMTP server manually with something like this: mailx -S smtp=x.x.x.x

Let me answer my own question. What i need to do was given in this topic: https://www.linuxquestions.org/quest...ateway-928331/

Add to the .mc file FEATURE(`delay_checks') and then run m4.
Restart Sendmail
Add to the access file the host from which i want to receive mail, localhost and all the other hosts, so:
192.168.1.100 RELAY
localhost RELAY
1 REJECT
2 REJECT
.
.
254 REJECT

Now with the delay_checks the authenticated users can send their emails from whatever host on the internet, non-authenticated users like spammers and other mailservers are blocked by the access list and my EFA server also non-authenticated is allowed by the access list. Tested it and works as i would expect.