tldr: has you noticed, sendemail has been removed and will not come back in future version.
Long story: https://forums.truenas.com/t/electriceel-24-10-1-issue
In this long thread there are some way to call new email function, but honestly at the time we struggled a bit… but If can help you too, i have made a small python to achieve what sendemail does before, here the link: oxyde1989/standalone-tn-send-email: stand alone truenas send email