Error "421 4.4.2 Connection dropped due to SocketError" when sending emails between authenticated SMTP servers.
Article ID: 155216
Updated On:
Products
Mail Security for Microsoft Exchange
Issue/Introduction
When Premium Antispam is enabled, connection timeout may occur and emails are queued on the sending server.
Cause
Connection timeout errors occur due to delay introduced into SMTP session for Premium Antispam scanning.
Resolution
Possible solution is to adjust the Maximum Transmission Unit (MTU) of sending SMTP server.
Lower the MTU value makes smaller packets. This helps improve the performance and avoid the timeout to occur. For details on how to configure it on Windows environment, contact Microsoft support for assistance.
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