More than one year of mail have disappeared and the sent folder is empty

Hey marrodan, 

Check to see if the messages show on the mail account’s website. This can isolate the issue.

Have you checked the trash? The missing mail could be there. You can read how to recover from the trash here:

1. Tap the Back button in the upper-left corner to view your mailboxes, then tap an account’s Trash mailbox.

2. Tap the email you want to recover.

3. Tap the Move Message button, then select a mailbox.

Delete and recover emails in Mail on iPad - Apple Support

Take care. 

Good afternoon marrodan,

It might be helpful for you to speak with an AppleCare technical advisor about this issue. Click the link below, choose the phone number for your region from the list, and call when it is convenient.

Contact Apple for support and service - Apple Support

Take care.