Now days users can send emails by hiding behind emails which they don't even own,
The email which was sent had an IP which is used only for doing frauds
http://whatismyipaddress.com/ip/46.167.245.116
https://www.abuseipdb.com/check/46.167.245.116
By seeing the softfail, looks like the above IP tried to verify itself as authorised sender but since it wasn't their on post4vps's TXT records(obviously), the sender wasn't completely authorised to use our email.
It's not MyBB side issue probably, since MyBB never shows user IP in user's profile it just allows other users to be able to send emails that through the forum, and I looked into he mail logs and looks like there was no such email sent from forum( MyBB ) side.