Inbound SMTP Restrictions

Inbound SMTP Restrictions

In order to provide the highest level of service our inbound processing infrastructure has some restrictions. Below are guidelines and limits you should follow to ensure the best results when submitting messages for delivery via SocketLabs.

-SocketLabs recommends configuring applications to open up to 20 simultaneous SMTP connections to

Numerous factors impact the absolute maximum number of connections available to customers at a given time. Customers with implementations that require more the 20 concurrent connections should contact SocketLabs Support. 

-Maximum message size, including any embedded images and attachments, is 40MB

-Maximum 500 total recipients per message

More Information

See our other articles for more information on related topics.

IP Addresses, Ports, and more inbound SMTP connection information: SMTP Connections

To better troubleshoot and diagnose SMTP connectivity issues see our blog on Diagnosing SMTP Service Connection Issues 

Related Errors

If your message is larger than 40MB you will receive the following error:
552 5.3.4 - Message too big for system

If you attempt to send to more than 500 addresses in one message you will receive the following error:
452 5.5.3 - Too many recipients

If you attempt to open an excessive number of connections you will receive the following error:
421 4.4.5 - Too many connections from your host