Step 1 Check the software version and configuration information.
Huawei Versatile Routing Platform Software
Software Version: USG2200 V300R001C10SPC100 (VRP (R) Software, Version 5.30)
Copyright (C) 2008-2014 Huawei Technologies Co., Ltd.
Secoway USG2210 uptime is 1 week, 1 day, 2 hours, 26 minutes
Step 2 Change the TCP-MSS value, but the problem persists.
[USG2200]firewall tcp-mss 1300
Warning: Configure tcp-mss will affect the system's normal service.
Step 3 Capture packets on the mail server. The analysis of captured packets shows that the email messages are rejected due to poor reputation.
Step 4 Query the IP address reputation on the Spamhaus website. It is found that x.246.10.x in the NAT address pool of the USG2210 is blacklisted.
Spamhaus is an international non-profit organization. Its major task is to trace spammers over the Internet and provide the spammer list to commercial spam filtering organizations.