It's clear to everyone (I hope) that storing passwords without at least salting/hashing them is a terrible idea.
What about emails? Let's say you keep the subscription email address, if you encrypt it properly it may not be usable to send emails to the users. On the other hand, if you don't encrypt it and the database gets stolen, all your users risk a potential spam.
This question is not about law-specific issues (although they may be given, they remain country-dependent) or about encrypting the database itself.
Best Answer
Storing a salted hash of the email addresses could be on option if you keep those records just for account confirmation / authentication.
In other cases, it seems to me that encrypting the emails would make the job of maintaining the database harder while gaining little in return.
Probably securing the access of the database itself is a better choice: usually there are a lot of other information in the database that you wouldn't like to be gathered.
A similar question on Stackoverflow: Is it worth encrypting email addresses in the database?