order to improve error
    handling.  We recognize the end of the mailbox list only at the end of the
    value or at a ';' (the group terminator).  This is so that we can turn
    invalid mailboxes into InvalidMailbox tokens and continue parsing any
    remaining valid mailboxes.  We also allow all mailbox entries to be null,
    and this condition is handled appropriately at a higher level.

    r