Email deleted on my iPhone is not synchronized with my other devices

The answer to that depends on the type of email account you have. For a POP account, email deleted on one device is just for that device, since POP mail is downloaded to the individual devices, not stored on the server. If it is an IMAP or Exchange account, mail is stored on the server, and synced to individual devices. What happens if you delete mail on the iPad, is it removed from the iPhone? If you go to Settings>Passwords & Accounts on both devices, select the mail account in question, on the next page what does it say just above the email address, POP or IMAP?