You can remove emails from your trash folder using the 'Empty Trash' action in the action menu. You can also choose to keep more recent emails using the 'Max age (days)' field in the empty trash window. For example if you choose a max age of 60 days, then only emails older than 60 days will be deleted.
Emptying the trash will permanently delete emails from your email server unless you turn off the 'Sync incoming mail' option in the edit account screen - advanced section.

You can also automatically delete older emails using the email folder setting to delete older emails. View. You will need to open the trash folder for each email account in the Folders section in the left side panel to change this setting.