Messages accumulate in the inbound queue when the URL Reputation essages begin queuing in the inbound queue. And large numbers of ERROR level messages are written to the bmserver logs.
The bmserver log may show these errors:
2016-02-05T00:41:06+01:00 (ERROR:2305.494909184):  DNS TXT query for "02eb395565cc52eb9d276c24cb567448.smg.ultra.brightmail.com" failed unexpectedly.
2016-02-05T00:41:06+01:00 (ERROR:2305.494909184):  Spamhunter module: Failure applying body regex rules.
2016-02-05T00:41:06+01:00 (WARNING:2305.660596480):  Latency threshold for feature "Symantec DNS URI Server: smg.ultra.brightmail.com" exceeded. Feature has been suspended.
The configured DNS servers for Messaging Gateway cannot process the additional load the URL Reputation Filtering generates. The URL Reputation Filtering was introduced in Messaging Gateway 10.6. This feature is enabled by default, which creates an increased load on the configured DNS servers immediately after the update. The URL reputation lookup added additional latency. This latency delays mail processing and messages queue in the inbound queue.
If DNS resolution is too slow to keep up with the URL Reputation checks, the following warnings may appear in the messages log file:
2021 Sep 5 08:57:18 (warning) named:  client 127.0.0.1#52011 (29f816e1d2c86de5cf218465f903b8d9.smg.ultra.brightmail.com): no more recursive clients (1000/0/1000): quota reached
2021 Sep 5 08:57:19 (warning) named:  client 127.0.0.1#52011 (1487575da5187efc309fd4f4b31b5dce.smg.ultra.brightmail.com): no more recursive clients (1000/0/1000): quota reached
Messaging Gateway 10.6.2 resolves an issue with correctly measuring the latency of URL Reputation queries. It now briefly disables URL reputation when excessive latency in the DNS queries is detected.
For earlier releases, this issue can be addressed by disabling the URL reputation service. Disable the server in the SMG Control Center Spam -> Scan Settings -> Enable URL Reputation filtering config option. This workaround should be considered a temporary fix until the issues with DNS are remedied.