Exchange Server 2013 – unable to send emails to certain companies, “421 4.4.2 Connection dropped due to SocketError.”

Error Message:
“Remote Server at email.xxx.com (xx.xx.xx.xx) returned ‘400 4.4.7 Message delayed’
1/21/2014 9:52:30 AM – Remote Server at email.xxx.com (xx.xx.xx.xx) returned ‘441 4.4.1 Error encountered while communicating with primary target IP address: “421 4.4.2 Connection dropped due to SocketError.” Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts. The last endpoint attempted was xx.xx.xx.xx:25′”

My solution:
Set-SendConnector “(connector name)” -ForceHELO $true
After changed ForceHELO value to true, I can see mails started to go out in Queue Viewer.
[http://www.networkadminsecrets.com/2010/04/421-and-451-exchange-2007-errors.html]

Other solutions [not suitable for everyone including me :P]
1) Change the MTU value:
– Run regedit, go to \H_L_M\SYSTEM\CurrentControlSet\services\Tcpip\Parameters\Interfaces
– Choose relevant network interface (e.g. check IP address, Default gateway, DNS)
– Create New DWORD named MTU with decimal value of 1400
[http://www.doitfixit.com/index.php?option=com_content&view=article&id=78:yahoo-emails421-442socketerror&catid=34:exchange-2010&Itemid=53]

2) Change IgnoreSTARTTLS value to true
Set-SendConnector “(connector name)” -IgnoreSTARTTLS $true
[http://forums.msexchange.org/m_1800546002/mpage_1/key_/tm.htm#1800546002]

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