Digital marketing is not limited to only SEO, but there are many methods of marketing like Email marketing, SMS marketing.
Most of the website owners, put a form on their website and ask people to enter their email-id for getting newsletters or updates. Unfortunately, this strategy doesn't work very well.
Lead magnet is something very popular now-a-days, including giving a free coupon, A cheat sheet of tips, A free webinar, A free quote, a consultation, A free trial, An eBook, A quiz, an assessment in exchange of an email-id from visitors.
Once you get the list ready, categorizing it is proven to work best part of email marketing, as it will improve the effectiveness of your email campaigns.
It is not a good practice to add emails from people's business cards that you collected at conferences. Unless you have express permission to send them your mails, they are technically spam otherwise.
Sending anything at all, would be a complete waste of your time and money if your message is not reaching the intended target.
When the same e-mail is sent across all the people who are present in the list on daily basis, it will not work. Rather, it is easy to get more engagement, by personalizing e-mails based on specific customer data.
Assume that, a company has a ton of useful and interesting content plus information, but readers will not be able to find that company unless they search for the specific thing. This problem can be solved by daily updating the readers with their own area of interest in which you are doing good.