Why can’t I see all my emails?
There is an option on your Apple device that sets the amount of emails to sync.
To make modifications on your “Mail Days to Sync” you will need to:
Open Settings > Passwords & Accounts > select email account > Mail Days to Sync > enter number of days > No Limit
Alternatively, Mail > Accounts or Mail, Contacts, Calendars, then follow after Passwords & Accounts.