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 email@example.com.
How to fix the sync error
Edit the email address of the contact so it is in the proper format of firstname.lastname@example.org, or delete the email address completely, then sync again.