cannot sync mac with new iPhone 8

Are you referring to the same mail account on both devices? If you check Settings>Passwords & Accounts, is the account set up exactly the same on both devices? Make sure after selecting the account on the first page that it says at the top, IMAP.

After you go to Settings>Passwords & Accounts, tap on the email account in question. The next page that opens should have the account name, and just above that, you should see the initials IMAP.