Thunderbird - When Autosave is Enabled - Autosave files are not deleted when the emal is sent

I am using Thunderbird version 38.8.0 on Ubuntu 16.04.

I have Autosave enabled, and set to "5 minutes".

I've noticed that when writing an email, if the email is written and sent within 5 minutes (or whatever period of time is set for Autosave), there is no Autosave copy of the email left remaining in the Drafts folder.

However, if the writing of the email takes longer than 5 minutes (or longer than whatever period of time is set for Autosave), then the Autosave copies of the email are left sitting in the Drafts Folder after the email is sent.

Is there a configuration setting in Thunderbird that ensures that Autosave copies of an email are deleted once the email has been sent ?

Thanks,

Roy