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, 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 makes emails send as separate messages.
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.
✅ Verify: Override for this step is disabled on follow-ups.
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.
✅ Verify: The Cc field is blank.
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.
✓ Follow-up steps do not include CC recipients.
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.
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.