I guess nobody else has seen this, which is kind of suprising. Somehow, perhaps not as part of the upgrade process, the data contained in dbo.aspnet_Membership.Email field got out of sync with the value for the same user contained in dbo.dnn_Users.  When I use the DNN UI to add or update a user's e-mail, it is done correctly in both database tables.  So I am going to write some SQL queries to fix it.

I used multiple steps as described by Will Strohl in the link below.

http://blog.strohlsitedesign.com/DnnUpgradePath.aspx

Will did a great job documenting frequent advice from Sebastian Leupold on the correct upgrade path should be from previous versions. Thanks Will!

In hindsight, I am thinking that the problem may have been present for some time, and nobody noticed until we audited the site after the upgrade.

The problem was that our dbo.DNN_Users table did not have the e-mail addresses in it but the dbo.aspnet_Membership table did.  If anyone else has this issue, here is the query I ran to fix it.

UPDATE

dbo.dnn_Users
SET dbo.dnn_Users.Email = dbo.vw_aspnet_MembershipUsers.Email
FROM dbo.dnn_Users
    INNER JOIN dbo.vw_aspnet_MembershipUsers
        ON dbo.dnn_Users.Username = dbo.vw_aspnet_MembershipUsers.UserName
WHERE dbo.dnn_Users.Email <> dbo.vw_aspnet_MembershipUsers.Email AND dbo.dnn_Users.Email = 'Please Update'

Our entries in the DNN UI Admin screen said "Please Update" for the user accounts missing the email address in the UI.