You can use the Maintenance pane to rebuild the mailboxes index in Mail.
This option can prevent and correct issues concerning incomplete, truncated, or missing messages.
You can also delete the existing index. The next time you open Mail, the application will ask you to re-import the emails, and the index will be automatically rebuilt.
If Mail is not responding, search is not functioning correctly, saving signatures is not possible, or the application won't open, this option can correct these issues.