How do I transfer mail to new iPad air

You would add the mail account to your device

settings, mail, accounts

and some will allow you to choose how far back you sync your mail, which will determine how much of your preexisting mail will transfer over (redownload from your mail provider’s servers)

How do I transfer my mail from my old iPad to my new iPad air

Just add your email accounts to the new device. Settings - Mail - Accounts - Add Account.