**Problem:**
When using Twenty Send Email UI IMAP/SMTP message threading is broken on
Twenty side as well as recipient email client
**Twenty side fix:**
- SMTP has no concept of `externalThreadId` sendEmail resolver always
returns null, this breaks threading
Fix is to pass `parentThreadExternalId` to
`resolveOutboundThreadExternalId` for SMTP/IMAP path
**Recipient email client fix:**
- Fetch associated threads as per RFC spec to write `References` header
```
From: johndoe@domain.com
To: janedoe@domain.com
Subject: Test
References: <root@...> <mid1@...> <parent@...>
```
