TL;DR: Keep the first email subject unchanged, leave follow-up subject fields completely empty, avoid overriding the sender or adding CCs on follow-up steps (unless you're using a forced sender with Thread behavior set to Reply, which manages CCs automatically), and verify the recipient sees replies grouped in one conversation thread.
Symptoms
Follow-up emails arrive as new conversations instead of replies.
Emails sent from the same campaign do not stay in one thread.
Changing the subject line causes a new email thread.
Using force sender or Override for this step on follow-ups sends emails as separate messages when Thread behavior is set to New thread (the default). To keep it threaded with a forced sender, set Thread behavior to Reply instead.
Adding CCs to follow-ups can break threading.
Environment
Applies to email sequences sent from lemlist.
Follow-up email steps now show an in-app warning when changing the reply subject would break threading.
Threading behavior depends on the recipient's email provider, such as Gmail or Outlook.
Step-by-Step Fix
Open the campaign sequence and check the first email step. Make sure the first email has the subject line you want recipients to see.
✅ Verify: The first step contains the subject you want recipients to see.
Open each follow-up email step in the sequence. If you see the in-app warning that changing the subject line will break the thread, keep the reply subject unchanged and do not edit it.
✅ Verify: The follow-up shows the original threaded reply subject and warns against changing it.
For every follow-up step, make sure Override for this step is turned off. Do not force a different sender for replies — unless you intentionally want a different sender on that step. In that case, enable Override sender and set Thread behavior to Reply to keep the email in the same thread.
✅ Verify: Override for this step is off, or Thread behavior is set to Reply if using a forced sender.
Leave the Cc field empty on follow-up steps. Do not add your own email address or any other CC recipients if you want the reply to stay threaded. Exception: if you're using a forced sender with Thread behavior set to Reply, lemlist automatically manages CC recipients to preserve the thread — this is expected.
✅ Verify: The Cc field has no manually added recipients (system-managed CCs in Reply mode are normal).
Send a test sequence to a mailbox you can access, then open the recipient mailbox and confirm the emails appear in one conversation thread.
✅ Verify: The follow-up appears as part of the original thread in the inbox.
Confirm It's Fixed
✓ Follow-up emails appear under the original email thread.
✓ The first email keeps its original subject line.
✓ Follow-up reply subjects are left unchanged.
✓ Override for this step is off for follow-ups, or Thread behavior is set to Reply if using a forced sender.
✓ Follow-up steps do not include manually added CC recipients (system-managed CCs in forced-sender Reply mode are expected).
Why It Happens
Email threading is controlled mainly by the recipient's email provider, but changing the reply subject, overriding the sender, or adding CC recipients can break the thread. lemlist now warns you in-app when changing a follow-up subject would break threading. Prevent this by keeping the first subject stable, leaving follow-up reply subjects unchanged, disabling sender override, and avoiding CCs. Exception: when using a forced sender with Thread behavior set to Reply, lemlist automatically adds both senders as CC to preserve the thread — this is intentional and does not cause broken threading.
Alternatives
If lemlist shows separate sent messages, check the recipient inbox before assuming threading failed.
Use Gmail or another accessible test mailbox to confirm whether the recipient actually sees one threaded conversation.
If threading still breaks, test with a different recipient email provider because Gmail and Outlook can handle threads differently.
Escalate If Unresolved
Share the subject line used in the first step and confirm whether the follow-up reply subject was edited.
Share whether Override for this step or CC was used, plus screenshots of the sequence settings and a recipient inbox view.
Learn More
Recipients may still see correctly threaded emails even when your Sent folder shows separate messages. A Gmail inbox is a good way to confirm whether follow-ups are correctly grouped into one conversation thread.