We don't always know what sort of e-mail clients the readers will use. There are a number of solutions I've seen people employ, some better than others.
Many websites ask the users to register in order to receive periodic e-mail newsletter.
When doing so, the users are asked if they want HTML or text mail. This means having to send out two separate bulk e-mails, but that shouldn't be much of a problem. This is by far, the cleanest solution. The downside to using this method is the fact that you must ask everyone what sort of e-mail they want. It also may mean having to keep that information in a database somewhere.