iPhone 4s mailbox question

You can create a new server stored mailbox with an IMAP or Exchange account, not a locally stored mailbox with a POP account.

With the account selected under Accounts, select Edit at the top right which will reveal a New Mailbox button at the bottom right.

Included with the iOS 5 user guide.

Get up to speed in the 21st century with an IMAP or Exchange account.

If you created a locally stored mailbox on the iPhone, mailboxes and email are not included with the iPhone's backup. If your iPhone were lost or stolen or damaged due to being dropped, kiss the locally stored mailboxes and messages goodbye. Or if you restored your iPhone with iTunes if wanted or needed, locally stored mailboxes and messages are lost, and the same when installing a major iOS update such as the recent iOS 5 update - the iPhone is completely erased as part of the install process.

Nothing is lost under any circumstances with an IMAP or Exchange account since you can store all account mailboxes on the server along with creating additional server stored mailboxes. All server stored mailboxes with an IMAP or Exchange account are kept synchronized with the server with each email client used to access the account - with an email client or via webmail access with a browser on your computer, and with the iPhone's Mail app, etc.

You can access a free Gmail account as an IMAP or Exchange account, which also provides Push access for received messages, and over the air syncing for contacts and calendar events.

A POP account is from the early ages of accessing an email account with an email client on a computer - when the overwhelming majority had access to a single computer only - well before there were any laptops much less smart phones and tablet computers.