1. Check Copies & Folders settings
Go to Menu → Account Settings → Copies & Folders and make sure “Place a copy in → Sent” is enabled and points to the correct Sent folder. If it’s pointing to a folder that doesn’t exist or can’t sync, Thunderbird will fail silently.
2. Large folders (10,000+ emails)
If the account has over 10,000 messages, Thunderbird may be trying to download and index all of them.
By default, IMAP accounts download both headers and full messages for offline use, which can create multi-gigabyte folders and cause:
- Sync delays
- Timeouts
- Folder corruption
- Failure to copy messages into Sent
To check or prevent this, go to:
Account Settings → Synchronization & Storage
Uncheck “Keep messages for this account on this computer” or limit sync to recent messages only.
Large folders also greatly increase the chance of corruption. Keeping the Inbox and Sent folders under about 10,000 messages each improves reliability.
3. Repair the Sent folder
Right-click the Sent folder → Properties → Repair Folder.
This rebuilds its index file.
4. Check for SMTP or syncing errors
If sending fails even briefly, Thunderbird never saves the Sent copy.
Look under Tools → Activity Manager for errors.
5. Antivirus interference
Mail-scanning features in antivirus programs can block Thunderbird from writing the Sent copy.
6. Disk space or profile problems
If the device is low on disk space — or the profile folder is very large — Thunderbird may fail to save messages.
Use File → Compact Folders to reclaim space.