Last week i upgraded to EE , and now i was bitten by the missing sendmail too.
I used winnie’s mail command above on DF , to send a “Bootup” & “Shutdown” e-mail. But no dice on EE.
I grabbed the “Oxyde” magic mail program from here (thnx) joe & oxyde:
https://github.com/oxyde1989/standalone-tn-send-email
chmod +x , and dumped it in /root
Initially i would have placed it in /usr/local/bin , but /usr is mounted ro 
As explained on the github page you’d need extra permissions to run the script successfully.
But as expected the “init/shutdown stuff runs at elevated privileges”
This “command” seems to work for me in the init/shutdown scripts
python3 /root/multireport_sendemail.py --subject "TrueNAS Bootup Notice" --to_address "nas-01@xxx" --mail_body_html "$(hostname) booted up at $(date)"
Edit:
I have just run one reboot cycle , where it worked on both boot & shutdown.
Maybe i was lucky, as I haven’t seen the issues mentioned here