Is there a difference between the email address I use to log in to the OkiOki app and the email address linked to OkiOki?

Yes. The email address you use to log in to OkiOki only gives you access to the app. This can be a personal email address or a general email address of your company. If you use a general email address you can 'share' a user and several people within the organization can work in OkiOki. Reminder emails are also sent to this email address for to-dos or unpaid invoices.

The email address that you link to OkiOki is the email address to which your business invoices are sent. In that mailbox, OkiOki looks for invoices that it will archive and link to the corresponding transaction.

Those two e-mail addresses do not necessarily have anything to do with each other.