Many thanks to the several folks who were kind enough to respond directly. Answers follow:
If you're using Koha to send overdue notices via email, how do you keep track of overdues where the patron has not provided an email address?
These notices are grouped together & emailed to the administrator address (KohaAdminEmailAddress setting under System Preferences). Simple macros in Word & Excel can be used to create a mail merge document so these notices can be mailed out.
If an email address is no longer valid, do returned messages get bounced to the administrator?
Bounced messages come back to the Administrator, or can be set to come to specific email addresses at the circulating branch in question (addresses for the branches are set under under Administration - Libraries and Groups). Finally, the text of the Overdue notices is set under Tools > Notices. Rules for sending the Notices are set under Tools > Overdue notice/ status triggers. Thanks to all again! Cab Vinton Sanbornton PL