QuickBooks Online sync error: Email Address does not conform to the syntax rules of RFC 822. Supplied value: XXXXXXXXXX

Why the sync error occurs

This error occurs when an email address for a contact in QuickBooks Online isn't in the proper format of address@domain.suffix.

How to fix the sync error

Edit the email address of the contact so it is in the proper format of address@domain.suffix, or delete the email address completely, then sync again.