How can I add a new mailbox?

1. You must be using IMAP

2. You have to add it from your email client, such as gmail.com

YOU CANNOT ADD A MAILBOX ON THE iPHONE ITSELF

I added my husband's email account to my iPhone this fall while we were away from home. Settings->Mail->Add Account->Other->then entered his settings for his account. Will this work for you?

I took Mailbox to mean folder. If not then do the above posters suggestion.

Sorry if I misread.

Did you get it to work?

New folder. Now that's a different story. I have mail folders on my MBP and none of them sync with my iPhone. Maybe in the next update.

If you are using the Mail application on your MBP, technically you create additional mailboxes although they appear as folders in the mailboxes drawer.

At the menu bar, you go to Mailbox and select New Mailbox.