The Email Hierarchy of Needs: Deliverability is the Foundation
This is the reason marketers use email service providers (ESPs) instead of sending emails via Outlook or Gmail. Brands also use ESPs instead of letting their developers with no email experience say, "we'll build it." Email delivery is complex.
The configuration of the mail transfer agent, the proper processing of bounces and unsubscribes, the feedback loops necessary to track and opt out spam complaints, and the proper throttle rates per domain takes a team. This is where the question "what is the best ESP" becomes interesting. All successful ESPs must have this piece down to a science. The first question I ask an emerging ESP is how many people are on its deliverability team. If the answer is "we all just pitch in" (that's a real answer I received once), then I stay away.
The single most important thing you have control of to optimize deliverability is good data practices. This means list hygiene and validation to eliminate malformed and undeliverable email addresses. It means opting out subscribers who ask to be unsubscribed. It means regularly mailing your entire list, having clean and transparent opt-in practices, and keeping your database clean and centralized to allow you to target subscribers based on their actions and preferences.
A terrible email message alone won't land your message in the spam folder, but it certainly won't help. Email can be marked as spam for a combination of things: content, IP reputation, from name/domain, etc. If you're spamming people, your email won't get delivered, even if your content doesn't have "FREE" or "Viagra" in it. If you send emails that people open and click on like crazy and nobody ever hits "this is spam," you can say free (almost) as much as you want. Most companies are somewhere in between. Test prior to sending. Usually one "free" won't kill your deliverability.