550 5.7.1 Command rejected SSL (def)

My friend cannot send an email to me because he is getting Failure notice with information:
... failed on DATA command.
Remote host said: 550 5.7.1 Command rejected SSL (def)

Is this means that there is something wrong with my mail server or his mail server?

When I check his domain with mxtoolbox.com this is what I found:
! SMTP Valid Hostname Reverse DNS is not a valid Hostname
! SMTP Banner Check Reverse DNS does not match SMTP Banner
! SMTP TLS Warning - Does not support TLS....

550 5.7.1 Command rejected SSL (def)