Customer has multiple address ID's. Defaults are set to: Bill to = ACCOUNTING, Statement to = STATEMENT.
In some cases they have the same person's email address. When setting up the Email for customer statements, the STATEMENT address ID is not available because the email addresses for ACCOUNTING and STATEMENT are the same.

For consistency, allow all address ID's with an associated email address be displayed in the Email setup on the customer card. Otherwise, if the statement email address changes, the user has to also open the email setup window to now change this or verify that the STATEMENT address ID is available.