Help FortiMail: Lost Input Channel

Hello,

I am new with FortiMail.

I have a question, we've got the log in mail event that show the message : "lost input channel from xxxxx.com [x.x.x.x] to SMTP_MTA after mail".

And when I check the same session ID, I've got the message : "from=<xxx@xxxxx.com>, size=126100, class="0", nrcpts=0, bodytype=8BITMIME, proto=ESMTP, daemon=SMTP_MTA, relay=xxxxx.com [x.x.x.x]".

And after that problem, the email sent by sender failed to receipt 

Only 2 logs in one session ID, and I don't know source of the problem.

And the problem also occurs in several mail inbound from different domain.

I run fortimail in VM as a Gateway with Firmware version: v6.0,build148,190422 (6.0.5 GA).

Could you help me to pointing the source of problem?