The following situation can happen if you are not careful.

You add a new user account to your store;

(in this case a generic one)

As you know each user is required to confirm their email address for it to be validated.  (they do this by clicking on an invitation in their inbox)

But due to your company's email wiring (email forwarding or aliases), the invitation ends up going to multiple or different email addresses, for example:

So Dave validates the invite, but from the wrong email address.  It ends up looking like this:

BRM does detect the mismatch and will indicate the Firebase ID in red.

The result is that Dave or will NOT be able to login.

The Fix is to delete the user account and re-add it.  Ensure the person invited does the accepting.

If in difficultly please contact BRM support