A mass emailing is valid and useful only if it's permission-based. However, you can send a first welcome email also to people who haven't given their permission yet, asking if they would like to subscribe to your newsletter.
You can do that using the following ready-made form and filling it with your information. Please note that this is a very general template, so do customize it as you like add any kind of relevant content that may help the reciepient to opt-in.
Subject: Brand New Offers
this is a welcome message by [Company name]. We have a wide range of products and offers about [your field of activity] and we constantly try to be one of the best companies in the field.
If you want to receive a periodic newsletter about our activities, please answer to this message to opt-in. We will not share your information with anyone else and you will be able to unsubscribe in any moment.
[Name and surname
If you choose not to subscribe, you will not receive other emails by us.