By default, the iOS Mail app deletes messages in your Trash folder after one week. Apple Mail sets this default to one month. To change these settings:
After adding your Fastmail account, you can change the setting for automatically removing deleted messages by going to:
- Tap Settings on the iPhone's Home screen.
- Go to Mail, then Accounts. For older iOS versions, tap Passwords & Accounts (or Mail, Contacts, Calendars).
- Tap your Fastmail account in the accounts list, then tap Account.
- Scroll to the bottom of the settings and tap Advanced.
- Tap Remove in the Deleted Messages section, then select Never.
- Open Mac Mail, and click Mail → Preferences.
- Select your Fastmail account from the accounts list.
- Click the Mailbox Behaviors tab.
- For “Erase junk messages:” select Never.