I updated mailspring to 1.0.10 by pulling the solus repo and running updatePackage.
Changelog: (From GitHub)
Features:
- A brand new signature editor makes it easy to create beautiful signatures with images and important contact details.
Performance:
- SQLite schema changes significantly improve Mailspring's sync performance on very large mailboxes.
Fixes:
- Mailspring no longer reports that it is still "looking for messages" in some cases after sync has completed.
- Search now correctly supports the "in:" syntax for all arbitrary folders and labels as well as built-in ones like "in:inbox".
- Several exceptions thrown in UI edge-cases have been fixed.