- check if you can still "Undo" the deleted message -- it looks like this
- re-check your entire emails, maybe you can still see fb email notifications aside from the fact that you filter it all already
- check your email's spam folder if the message still exist (depends on the setting i.e. auto delete, delete in 1 week etc.)
- if the message is important, how about asking the sender to send the message again? not unless it's an evidence to something
- deleted message can be recovered in a span of time.. if you can't undo it, then no luck at all. Even contacting fb support.