Obviously, you have to followup with your prospects who have opt-in and who have given you their name and email address.
After all, with your lists of database of prospects and paid customers, you will be able to build a better relationship with them and get them to buy again and again in the future when you launch your new products and services.
You can followup with your correspondence and engagement with those who have yet to buy from you by constantly and regularly supplying them with useful information and update on your product and service.
The online advertising landscape have changed tremendously that have presented great challenges to email marketer not to spasm in their emails marketing campaign. You don't have to worry if you send your emails effectively and gather opt-ins that follow the guidelines.
Be mindful, don't just send an email and say "Here's the secret to make more money the lazy way...buy now or you'll regret it..." That would tantamount to marketing suicide.
Give yourself a chance to engage and converse with your audience through your mailing list via your email marketing. Give them valuable content so as to make them feel comfortable to do business with you.
Present yourself to them as an Expert in your niche and industry. Make yourself useful to them, make them feel like you are one of them who feel their pain, understand their problem and know their predicament.
Review your products or offers thoroughly and write in a way that shows them why it's beneficial and not so much on the description of the products or services itself.
Provide testimonials and stories that people have benefited from using your products. After all, your prospects would think that if it has benefited other people, it would also definitely benefit them too, right?