how do I sync mail from my iphone to my ipad

You cannot sync directly between an iPhone and an iPad. If the mail was downloaded on one device (i.e. the email account is POP), you'll need to sync the messages to your computer, from which you can then sync to the other device.

Regards/

On the iPad and iPhone configure the email account as IMAP (not POP3) so that all the messages are stored on the server, then the email account on both devices will be in sync.

How do you configure your account from pop3 to imap

Changing from POP3 to IMAP is a function of your email provider. You need to check with them to see if they even offer IMAP. Some do and some do not. If they do not your choices are to 1) configure email on the PC/iPad to keep Mail on the mail server or 2) change email providers to something like Gmail or Outlook.com (as examples).

This may help if you do not want to or can not switch to IMAP...

Using a POP account with more than one device - Apple Support

You can use imap server settings to download to both devices, this ensures that anything you do one one device will be reflected on the other when you open your mail app.