View an email in Mail on iPhone
In the Mail app 
, you can preview the contents of an email without opening it.
On models with 3D Touch, press a preview to peek at it, then swipe up, left, or right to use quick actions. Press a little deeper to pop the email open. See Use 3D Touch for previews and shortcuts on iPhone.
Show a longer preview
Go to Settings 
> Mail > Preview, then choose up to five lines.
Show the whole email
In the list of emails, tap the one you want to read.
Show the whole conversation
Go to Settings > Mail, then turn on Organize by Thread.
To change settings for how threads are displayed—such as Collapse Read Messages or Most Recent Message on Top—go to Settings > Mail. You can swipe an email left or right in a thread to reveal actions, such as Reply, Flag, and Trash. To change the swipe settings, go to Settings > Mail > Swipe Options.
Show who else received an email
While viewing the email, tap Details at the top of the email.
If you don’t see Details, tap Hide to show it.
Show To and Cc labels in your Inbox
Go to Settings > Mail, then turn on Show To/Cc Labels.
You can also use the To/Cc mailbox, which gathers all mail addressed to you. To show or hide it, tap Mailboxes, then tap Edit.
Add someone to Contacts or make them a VIP
Tap the person’s name or email address, then tap Add to VIP. You can also add the address to a new or existing contact.