Some versions of the Symantec Messaging Gateway ("SMG") MTA do not scale well as the fraction of messages accepted and delivered with TLS increases approaches 100%.
As the fraction of messages delivered with TLS increases beyond 90%, message acceptance begins to slow and delivery slows as well with the queue reporting large numbers of messages in the active delivery state. CPU is not depleted in this state although a single core may be at 100%.
Earlier versions of the SMG mail server were dependent on a single thread to perform TLS negotiation. As TLS delivery approaches 100%, this limitation increasingly acted as a bottleneck for mail acceptance and delivery.
This issue has been addressed with Symantec Messaging Gateway 10.6.0.
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