The only security settings for Mail are in the accounts settings. Go to Settings > Mail, Contacts, Calendars and choose your account. Tap on the Account item. The exact locations and availability of security settings depend on the account type, so you may have to dig around a bit. Besides the password, the only two settings you are likely to find are SSL and S/MIME. The first needs to be on or off depending on your provider's requirements. I haven't the foggiest idea of how to set up and use S/MIME.