How to set up email on Intranet - no ISP, no internet, no encryption, no spam filters; everything behind my firewall

The situation is a Raspberry Pi which is hosting several services. Sometimes, a service may fail, then I need to send an alert to my desktop. I have done many web searches, most of the results talk about connecting to an external email service, so are not relevant to me. The desktop has Linux Mint 22. Mail and postfix seem to be already installed. There is a log file (/var/log/mail.log) which only has start-up messages. The Pi (3B with Raspian 12 bookworm) is headless. I have installed sendmail, ssmtp and postfix. I cannot find a mail log file. I have found files /var/mail/NAME (where NAME can be the sender [the Pi user] or the receiver [the Mint user]). The contents seem to show that the test messages were processed correctly, but I did once see Diagnostic-Code: X-Postfix; delivery via local: delivers to command: procmail -a "$EXTENSION". I get no error when running this on the Pi sendmail -v ME@MYMINT It simply says Mail Delivery Status Report will be mailed to [user]. mailq shows nothing. On the Pi, the file '/etc/postfix/main.cf' contains smtpd_banner = $myhostname ESMTP $mail_name (Raspbian) biff = no append_dot_mydomain = no readme_directory = no compatibility_level = 3.6 # TLS parameters smtpd_tls_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem smtpd_tls_key_file=/etc/ssl/private/ssl-cert-snakeoil.key smtpd_tls_security_level=maysmtp_tls_session_cache_database = btree:${data_directory}/smtp_scache smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination myhostname = MYPI.MYDOMAIN.org myorigin = MYPI alias_maps = hash:/etc/aliases alias_database = hash:/etc/aliases mydestination = $myhostname, MAIL@MYDOMAIN.org, localhost.localdomain, localhost, MYMINT.MYDOMAIN.org relayhost = mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128 192.168.1.0/28 mailbox_command = procmail -a "$EXTENSION" mailbox_size_limit = 0 recipient_delimiter = + inet_interfaces = all inet_protocols = all MYDOMAIN is a made-up name that is only used for this project. MYPI and MYMINT are the host names of my boxes, with IP addresses in the range 192.168.1.x . Can you help me set up email between these 2 boxes. I think I also need help in reading mail on the Pi. **Edit - Firewall considerations** I have tried to eliminate any firewall issues. I have opened port 25 on the Mint box. There is no firewall on the Pi.