Q When I updated my wife’s laptop to Yosemite, Mail stopped working. Its sidebar reports several unread messages, but none is listed for reading. How can she read those messages?
A Before doing anything else, make sure that her Mail folder is well backed up, preferably not just using Time Machine but by making a copy of it from the Library folder in her Home folder.
With the mail service offline, open the Activity window via the Window menu. Try to connect to the mail service, watching that window for clues. You may also find the Connection Doctor helpful to eliminate basic issues.
Next check carefully through the account settings in Mail’s Preferences, comparing against working settings or those advised by your mail service provider. You should then be confident that the connection is correct, and can focus on the mailbox itself. Using the last command in the Mailbox menu, try rebuilding the mailbox.
If all those fail and she is running the latest version of Yosemite, try re-installing it through the App Store, then bringing everything fully up to date. Although that will be quite time-consuming it should fix any problems which could have arisen in the original installation.
Updated from the original, which was submitted for publication in MacUser volume 31 issue 3, 2015, but never appeared.