Why are my emails not threaded?

Providers such as Gmail and Microsoft Exchange / Office 365 accounts handle threading, by default they manage this by the message Subject.

Nylas can assist in threading, when sending vie our API, with the in_reply_to, by passing this attribute it assist the provider in determining a relationship between messages.


For Gmail users: Gmail overrides the headers of emails sent by Nylas for Gmail accounts and does not alert us of this change. We pick up the change after a few minutes. But this can disrupt threading if the user tries to reply to their own outbound email using our API within a few minutes of sending their 1st email. We typically only see this from test cases.  Please create a support ticket if your users need this sort of sending behavior.


If messages are not being threaded then please inspect the headers of all the messages, and ensure that there is a relationship between them:

  • First message has no in_reply_to
  • Second messaged uses the Header Message-ID of the first
  • Third message uses the Header Message-ID of the first or second


Nylas only syncs the messages and thread information as per the provider, if it's not threaded on the provider then it's a provider issue. If it's threaded on the provider but NOT on Nylas, then reach out to support.


For IMAP accounts only, Nylas handles email threading. Please create a support ticket for us to investigate further.




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