Error "421 4.4.2 Connection dropped due to SocketError" when sending emails between authenticated SMTP servers.
Last Updated May 19, 2014
When Premium Antispam is enabled, connection timeout may occur and emails are queued on the sending server.
Connection timeout errors occur due to delay introduced into SMTP session for Premium Antispam scanning.
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.
Another workaround solution is to disable Premium Antispam scanning between authenticated SMTP servers.
In SMSMSE 6.5.1 and earlier, Antispam scanning on authenticated SMTP sessions is disabled by default, these steps do not need to be taken if you are running 6.5.1 or earlier.
WARNING: It is highly recommend that you back up your system registry before making any changes. Incorrect changes to your registry could result in permanent data loss or corrupted files. Modify only the keys that are specified. See Backing up the Windows registry before proceeding.
To edit the registry:
1. Exit all open programs
2. On the Windows taskbar, click Start > Run
3. In the Open box, type the following text: regedit and click OK
4. In the Registry Editor, create the following key: