Google keeps deleted messages in your trash label/folder for 30 days.
The best way to locate an email that you think you deleted is to login to the webclient and use the seach box. Once you search look for this text at the bottom of the message list:
You can also quickly go straight to your trash label by searching for
In Thunderbird, look in your [Gmail] > Trash folder. Also check your [Gmail] > All mail folder, some clients simply remove labels instead of moving a message to your trash.
Email emptied from the trash folder is not recoverable!
With the ample storage that UNI Gmail provides, it is recommended that you generally never empty your Trash. Trash folder messages are automatically deleted after 30 days so this will provide you a 30 day window of time to recover a message that was accidentally deleted. Once a message is deleted from Trash, it is gone forever.