What should I do if my email is being throttled by mailbox providers?

When your email is being throttled by mailbox providers, there are a few steps you can take. In most cases of throttling, mailbox providers send soft bounce error messages back to the sender, indicating that they are limiting the amount of email they will accept. This may occur due to:

  • Reputation issues with an IP address or domain
  • Sending from a new IP address or domain
  • Sending too much email in a short period of time

In some cases, senders may not receive SMTP soft bounce error messages and will only discover throttling when they notice the slow delivery of their email or messages accumulating in their email deployment queue.

What to do
If email is being throttled by mailbox providers, reduce sending volume and frequency, if possible, by: 

  • Adjusting connection and throughput settings:
    • Open up fewer simultaneous connections.
    • Lower the sending volume per hour.
    • Lower the sending volume per minute.
  • Making other adjustments:
    • Temporarily send email to your most active and engaged subscribers (identify which ones have open and click activity; this may help improve your reputation with the mailbox provider).
    • Send email in smaller batches rather than as one bulk send.
    • Increase the length of time between retries (for example, from five minutes to one hour) and limit the number of retries.

Talk to your email administrator or email service provider about making adjustments to ensure that the adjustments can be supported. Then allow enough time to measure the impact of your initial adjustments before making additional ones. 

Once you have addressed the cause of throttling, you may have a full deployment queue. Clear the queue slowly so you don't flood mailbox providers. A surge in email volume may be perceived as spam and undo all of your recent work.

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