An email newsletter is defined by valuable content, and this is what subscribers will expect. If somebody subscribes to a newsletter, she does not expect product offers without context, or a magazine ad turning up in her Inbox.
If a "newsletter" consists of nothing but an ad for a digital camera, it will quickly find its way to the trash.
If the newsletter contains an interview with a great photographer, some tips and tricks, and a detailed guide of a photographic technique that you have just the perfect camera for, for example, then the subscriber might get interested.
