The most important people can be added as VIPs, to help differentiate them from the rest of your contacts.
At first there are no VIP contacts. To add some, open the Mail app and tap the VIP mailbox followed by Add VIP. Now, tap on a contact from the list to select them. To add more contacts tap Add VIP again. You can add as many contacts as you’d like.
After a few have been added, you can set a custom tone for when emails are received from these people by tapping VIP Alerts. This takes you to the Notifications settings. From here, select Mail followed by the VIP option. Change the Alert Sound so it’s clear who it’s from when a VIP email arrives.