You have upgraded your Symantec Messaging Gateway (SMG) to version 9.5. After the upgrade you notice that the scanners are deferring inbound messages giving "452 4.5.3 Service unavailable, batch limit reached." as the reason. If the SMG deployment is downstream of another SMG deployment or another Mail Transfer Agent (MTA) that is accepting e-mail from the Internet; then, you are likely seeing messages building up in the delivery queue of MTA upstream of the SMG that exhibiting this behavior.
452 4.5.3 Service unavailable, batch limit reached.
In version 9.5 a new MTA configuration parameter, "Maximum number of messages per session" was introduced. If Connection Classification is not enabled, the default value for this setting, for the inbound MTA settings, is set to 100. The default value of 100 may not meet the needs of your environment and as a result the SMG is deferring inbound messages in an undesirable manner.
Increase the value for the "Maximum number of messages per session" SMTP parameter for the inbound MTA settings to a number suitable for your environment.
Environments where the SMG deployment does not accept e-mail directly from the Internet are most likely to be affected by this. Some examples include a hosted mail solution, multiple SMG layers, any MTA(s) upstream of the SMG deployment that will send multiple messages over a single connection.