Context

When sending email sequences with multiple messages, sometimes the second or subsequent emails may appear as new threads instead of being properly threaded with the original email conversation.

Answer

If your follow-up emails in a sequence are not threading correctly when received, this is often due to the first email being filtered to the spam folder by the recipient's email client. When this happens, the subsequent emails cannot properly thread with the original message since it's not in the primary inbox.

To ensure proper email threading:

1. Ask recipients to check their spam folder for the first email and mark it as "Not Spam"

2. Add your sending domain to their safe senders list

3. Monitor your domain's spam score and email deliverability metrics in your account settings

Note that while emails may appear unthreaded in the recipient's inbox, they should still show as properly threaded within your application dashboard.