“Messages queued (dynamic flow control)” encountered during email delivery

Learn the cause and ways to resolve the issue where you get the “Messages queued (dynamic flow control)” error message during email delivery.

Description description


  • Campaign
  • Campaign Classic


You have collated the error messages for the most common domains that are having deliverability issues with your emails. The most volume is associated with the following error:  Messages queued (dynamic flow control).

Resolution resolution

  1. Dynamic flow control errors occur when the Mail Transfer Agent (MTA) has too many errors in a domain in a brief period. The MTA is coded to enter this backoff mode to prevent a blocking/throttling event in a domain from worsening, which could very well happen if it doesn’t slow down or stop sending temporarily.

  2. And to be clear, the term errors could mean any failed attempt to connect or deliver to a domain. It could be an internal MTA error, not just a (Simple Mail Transfer Protocol) SMTP bounce or a connection attempt refusal from the domain. So, this needs to be investigated to make sure that the issues aren’t with the MTA itself.

  3. Assume the Dynamic flow control errors are occurring in just a few domains. In that case, it’s more likely that it’s due to Deliverability reputation and that they’re being blocked/throttled by those domains. And if it’s a Deliverability reputation issue, you will need the Deliverability hours to investigate the cause and determine a strategy to fix things.