To ensure your follow-up emails stay in the same thread as your initial email, two conditions must be met.
Subject Line
Leave the subject line of your follow-up steps empty to automatically carry over the subject line from the previous step, or manually enter the same subject line.
If the subject line differs from the previous step, the follow-up will be sent as a separate email in a new thread. You can intentionally change the subject line if you want a follow-up to start a new thread.
Sending Account
Instantly sends all follow-ups from the same sending account by default.
If you remove that account from the campaign, leads will receive follow-ups from another active account, which will create a new thread.
Note: Depending on the recipient's email provider, "RE:" may be automatically added to the subject line of follow-up emails.