Too many concurrent connections opened.

When making an POST query that hits the provider it returns the response:

Too many concurrent connections opened.


Endpoint / Log: 

  • Any endpoint that hits the provider i.e. /send or /search


  • This is a provider response, Nylas forwards this response.
  • This is caused by:
    • Services such as Nylas connecting to the users Mailbox, we create at max 2 connections.
    • Delegated / shared / subscribing mailbox access
  • Providers can define these values, they have no static value. Reach out to the mail administrator to have this limit increased.
  • From Microsoft
EWSMaxConcurrency Exchange 2010 Exchange 2013 Exchange Online Defines the number of concurrent open connections that a specific user can have against an Exchange server that is using EWS at one time. The default value for Exchange 2010 is 10. The default value for Exchange 2013 and Exchange Online is 27. This policy applies to all operations except for streaming notifications. Streaming notifications use the HangingConnectionLimit to indicate the number of open streaming event connections that are available. For more information, see What throttling values do I need to take into consideration?.



  • The user would need to reduce the number of services connecting to the mailbox. Audit connected applications.
  • Great article on how to reduce concurrent connections via Outlook / Live.



Developer / mail administrator article on how to reduce the impact in Microsoft environments

Microsoft Mailbox limits 



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