E-mails are actually a great means of advertising: You can make contact with your customers more or less for free, send them information which is up-to-the-minute and offer them a chance to give a direct reaction. All of this requires relatively little effort. Almost anyone can create an e-mail newsletter. However, you will need a little support from a graphic designer and a programmer.
However, it is precisely because they are so low in cost that e-mails are also often used by disreputable suppliers as a means of advertising. We all know this as spam. Nobody likes receiving spam, so you should be careful when using this medium.
It is therefore recommended that, as a rule, you only send e-mails to existing customers. Even stricter rules apply to newsletters: Legally, you are only allowed to send newsletters to recipients who have given their express permission for you to do so. However, you can also obtain this permission online by writing your existing customers an e-mail where you offer them the opportunity to receive the newsletter.
However, none of this means that newsletters are a fundamentally bad means of advertising. On the contrary: Many customers want to receive up-to-date information and like it when you draw their attention to special offers, for example.