SMTP client send QUIT command before waitting for OKAY from server

There's a smtp issue here and it's weired. Our business partner failed to send email to us lately. After checking, we found our partner's smtp server sent QUIT command immediately after email content, before waiting for 250 OKAY from us. It causes the connection shutdown and our smtp server thinks the transaction is incomplete and drops the email.

It looks like against the RFC standard of SMTP protocol. Who has experienced this before? Thanks in advance!

enter image description here

How many English words
do you know?
Test your English vocabulary size, and measure
how many words do you know
Online Test
Powered by Examplum