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

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

Description description


  • Campaign
  • Campaign Classic


The customer has collated the error messages for the most common domains we are having deliverability issues with for their client 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 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 an SMTP bounce or a connection attempt refusal from the domain. So, the customer will need to investigate this on their end and make sure they aren’t having issues 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, the customer will need Deliverability hours for someone to help them investigate the cause and determine a strategy to fix things.