If have deleted the emails and cleared the Deleted / Trash folder then you may have likely deleted the emails permanently.
You can try visiting the email provider’s website (for example, gmail.com for a gmail account etc) and check whether those emails exist there in the Trash folder and then retrieve them. NOTE that if this email account is an IMAP account then all emails that you delete on your iPhone will be deleted on the website as well.
If the email account was a POP account and you had those emails saved on your iPhone AND created a backup of your iPhone on a computer, then you can restore your iPhone from that backup and return the state of your iPhone to the date and time that the backup was created.
Axel F.