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SMTP and REST API Injection Best Practices for Improved Performance

Last updated June 2021

SparkPost wants to make sure all of our customers have the best sending experience through our service.  We have a few recommendations to ensure you are injecting in the most efficient manner and to optimize overall message throughput.

Sending via the Transmission REST API

We recommend that you send via the Transmission REST API, if possible. Overall the REST API is more efficient than SMTP and requires less resources on the client side.

  • On timeout or any 5XX error the best practice is to retry.

  • When retrying, we recommend that you leave at least 5 seconds before the first retry, then use exponential backoff.

  • Set API timeout to 300 seconds.

  • There is a 300MB limit on payload size for each transmission, but we recommend keeping a payload size no greater than 50MB.  This refers to the total size of the JSON used for transmission, including the content, recipients and any substitution data.

  • Use concurrent connections by having multiple transmissions running in parallel.

  • Send smaller concurrent batches.  Sending 10 transmissions of 2,000 recipients each is more efficient than sending 4 transmissions of 5,000 recipients.

  • For large bulk mailings of 100,000 or more, keep the maximum number of recipients to 10,000 and the minimum number of recipients to 2,000 per transmission call.

  • Send up to a maximum of 10,000 transmissions per connection.

  • Use a connection for up to a maximum of 15 minutes, before closing it and creating a new connection.

  • Respect the DNS TTLs for the API endpoint's hostnames. We use load balancers. The load balancers do not have a fixed set of IP addresses. IP addresses can be added and removed automatically during scale out / scale in. We recommend that your applications look up the hostname in DNS every time a new connnection is made.

  • Limit the concurrent connections to a maximum of 10.  (For SparkPost Enterprise customers, please contact your TAM for guidance on the number of concurrent connections.)

Sending via SMTP

Follow these best practices in order to optimize SMTP injection rates (and overall sending speed):

  • Use concurrent connections by having multiple batches running simultaneously - use up to 10 connections at a time.  Having multiple connections open and running allows more messages to be sent in a shorter period of time.  (For SparkPost Enterprise customers, please contact your TAM for guidance on the number of concurrent connections.)
  • Keep the TCP connections open between messages.  Closing and opening connections on each message is much less efficient and can slow performance. Best practice is to reuse connections for no more than 100 messages and then create a new connection. This allows local and remote resources to be cleaned up efficiently.
  • Ensure your SMTP client has pipelining enabled, to reduce the roundtrip delay on some SMTP commands.
  • SparkPost strongly recommends using TLS with SMTP to protect your message content, recipient information and API keys in transmission.
  • If TLS is not supported by your application, SparkPost recommends using API keys with only the "Send via SMTP" privilege enabled. It is also good practice to regularly cycle your API keys to limit exposure of keys sent in the clear.

Following these best practices for optimal use of SMTP injection:

  • On timeout or any 4XX error the best practice is to retry.
  • Use a timeout of at least 30 seconds for the SMTP authentication phase (AUTH command).
  • General recommendations for SMTP client timeouts can be found in section of RFC 5321

Note on International Performance

"Chatty" protocols, like SMTP, with many round trips between client and server, may be subject to higher than expected latency. We recommend using our REST API in performance sensitive situations. If you need to send via SMTP, the closer your servers are to ours the better. SparkPost is hosted in the US, so sending from a server in North America is recommended. For SparkPost EU, sending from Western Europe is recommended.

General Message Size Considerations

  • For optimal deliverability, the total size of each message should not exceed 100kb.
  • For messages with inline images and attachments, keep the total message size to less than 20MB - total size limit includes the body PLUS images PLUS attachments

Need help or advice with technical issues?  Check out our community Slack channel at or submit a support request.

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