I'm pretty sure it takes a while for an email to be properly deleted in most online email systems, as it will have to flush through replicas, backups, etc. for a while.

Some metadata about emails may also be stored permanently out of the email system.

There is no guarantee that an email service provider deletes all copies of an email when you press 'delete' on the email, but most likely after a while the email will not be recoverable.

It _really_ depends on so many things.