Messages are disappearing from my Trash

By default, iOS deletes messages permanently after they've been in the Trash for a week. To change this behaviour:

  1. Open the Settings app.
  2. Tap Mail.
  3. In Accounts, tap your Fastmail account. It will be labelled with whatever you typed in for the Description field when you set it up.
  4. Tap Account, scroll to the bottom and tap Advanced.
  5. Under Deleted Messages, tap Remove and change the setting from "After one week" to "Never".
  6. Tap to go back to your Account, then tap Done.