Got the bounce email back: Sophos Firewall was unable to send the following mail

All,

We got the several mail bounce back mentioned at title with additional info:

Internal Server error

Timeout waiting for client input

This is happen within last 2 weeks. Several solution has been made until restarting the device, but no luck.

But, this mail bounce back error only to several domain, not all and looks like random, to gmail.com was ok, to yahoo.com was not ok (simple check sending mail)

Any other solutions ?

This is critical situation that impact to our business. 

FYI,

Mail server: IBM Domino 9

XG Firewall Firmware: XG310 (SFOS 17.1.3 MR-3)

Thank you,

Regards,

Rachman