mail( $to, $subject, $body, $headers, '-f' . $from );
// В $headers есть заголовки: From, Reply-To, Return-Path

Server:         8.8.8.8
Address:        8.8.8.8#53

Non-authoritative answer:
666.666.666.666.in-addr.arpa       name = mysite.com.

Authoritative answers can be found from:

Apt-get install exim4

nano /etc/exim4/update-exim4.conf.conf

dc_eximconfig_configtype='smarthost'
dc_other_hostnames='server'
dc_smarthost='smtp.yandex.ru::587'

nano /etc/exim4/passwd.client

smtp.yandex.ru:username@yandex.ru:password

nano /etc/email-addresses

root: username@yandex.ru
www-data: username@yandex.ru

sudo /etc/init.d/exim4 restart

@reboot root sleep 60 && echo "Server reboot" | mail -s "server reboot" mail@yandex.ru

# cat testmail.php


<?php
mail('komu@mail.ru', 'тема письма', 'тело письма', 'From: ot.kogo@mail.ru');
?>


# php testmail.php