a suggestion on how could could bypass these problems.
Eg: user John (uname=john123) is registered with email "oldJohnMail@yahoo.com"
He could go to a page, enter his username and get something like this (mixed dummy and real emails, splitted):
He can easily determine his own mail address but you don't really violate privacy either, as you don't say which mail address belongs to who.
If you think it is too obvious, you could salt the data, so for example "olXJohnXail" instead of oldJohnMail. It will still be fairly easy to see it for the actual user, and almost impossble for spambots etc
Anyway, just a suggestion