How do email threads/chains work, and how do I ensure sending two emails separately will be merged together in a thread in the users email client?

Question

I'm using AWS SES to send emails to customers. I want to send an initial email to confirm an action they've made on my website, and then send subsequent emails to that same email address to notify of any subsequent activity on that initial action.

Different email clients appear to implement this behaviour differently, and I've read about the thread-index header here, but that doesn't cover all clients.

Is there a standard way to mark that an email belongs to the thread of a previously sent email?

Thanks

Answer 1

When adding the References Header to Emails, you can add message-ids of previously sent emails in order to create email threads.

Of course it's again a topic of the client to fulfill this feature, but it should be supported by major email clients.

Heres an old blogpost about that. (considering that email is also old, it should be fine ;) )

https://wesmorgan.blogspot.com/2012/07/understanding-email-headers-part-ii.html