My iPhone has recently started deleting my email automatically from my Phone after a couple of weeks. I am wondering how exactly I can keep all emails that come to my phone as I need them.
If the account is associated with Outlook or similar, it has a feature called "mail days to sync". To increase to show all emails, open settings, go to Mail, tap on the offending account, then change mail days to sync from the default amount to the longest period of time. This will restore all previous emails that have gone missing, and keep any new ones.