I have some software I wrote that sends automated notifications to a specific email address. I'd like GMail to group notifications with the same subject line together into a single conversation, but for some reason it is treating each of these emails as a new conversation (i.e. listing them separately in the GMail UI). I had thought using the same subject line would be enough.

Right now, I'm sending emails with no additional special headers. My question is: Are there extra headers I can include in an email (such as a "conversation ID" or something) that will encourage GMail to group messages into a conversation (assuming the subject lines are all the same)?

If not, is there some other condition that GMail uses to group messages into a conversation that I am missing here?

1 Answer 1


I guess my search-fu was off, I found my answer with some further research shortly after posting.

Looks like as of 2019, emails require a References header referencing the previous message's Message-ID in order to be grouped into a conversation.

This is also described in the GMail help article about conversation grouping:

Emails are grouped if each message meets the following:

  • The same recipients, senders, or subject as a previous message
  • A reference header with the same IDs as a previous message
  • Sent within one week of a previous message

So the solution is to keep track of the Message-ID when I send a notification email, and then set References to that message ID in the next email.

I've implemented this and can confirm it works, and GMail groups the emails into conversations.

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