Hello and thank you for your reply.
So here are the logs when I tried to send an email from a PHP script to a hotmail address:
Nov 15 03:57:30 {HOSTNAME} plesk-sendmail[1222811]: S1222811: from=<
[email protected]> to=<*******@hotmail.com>
Nov 15 03:57:30 {HOSTNAME} plesk-sendmail[1222812]...