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421 4.4.5 - Too many connections from your host

To ensure all of our customers can get the maximum possible performance out of our system, we may reject SMTP connections from organizations opening more than 20 concurrent SMTP connections.

If you are using a system that requires frequent sending, such as sending transactional messages, we recommend that you configure your systems to make use of a local MTA that can relay to us as a Smart Host. Doing so can more efficiently manage the connections you have open to us by automatically queueing messages as well as sending multiple messages with each open connection.

Please review the following article for more information on setting us up as a Smart Host for mail relay:
How to increase SMTP performance when sending messages through Socketlabs

Customers on Enterprise tier service plans may contact their account managers to discuss customized concurrent SMTP connection limits.