how can l retrieve an email?

I've never heard of any ISP "charging" for copies of emails? They keep them for a set amount of time (as specified in their terms) where you can access them via IMAP or online/their website and then they are deleted.

If you haven't emptied the trash, you can retrieve it from there.

Or, from your backup you made before you deleted it.

If you have an email host provider, they will usually keep copies for 30 days or longer, but they wouldlikely charge.

I hope that works