What are the main factors that affect email deliverability?

You can set up perfect authentication, send from a pristine IP, and still land in spam if your subscribers don't want what you're sending. That's because email deliverability isn't one thing. It's four pillars working together, and if one breaks, the whole structure wobbles.

Authentication proves you're really you. Mailbox providers want to know the email claiming to be from your domain actually came from your servers, not a spammer pretending to be you. That's what SPF, DKIM, and DMARC do. Without authentication, Gmail and Outlook treat you like an unknown sender. With it, you're at least verified.

Reputation is your sending history score. Every mailbox provider tracks how many of your emails bounce, how many get marked as spam, and whether you're consistent or erratic. High bounce rates signal you don't maintain your list. High complaint rates signal you're annoying people. Sudden volume spikes look suspicious. Your reputation lives at two levels: your sending IP address and your domain. Mess up either one and deliverability tanks.

Engagement tells mailbox providers whether their users actually want your mail. If people open your emails, click links, move them out of spam, or reply, that's a green flag. If they ignore you for months, delete without opening, or mark you as spam, that's a red flag. Engagement matters more than almost anything else because it's the most direct signal of value. (You can't fake it, either. Mailbox providers see through click farms and fake opens.)

List hygiene means you're only sending to people who signed up, still want your email, and have valid addresses. Sending to old lists full of abandoned addresses, spam traps, and people who forgot they subscribed is the fastest way to destroy your reputation. Good hygiene = explicit permission, regular cleaning, and removing unengaged subscribers before they become a problem.

Here's the catch: these four pillars don't work in isolation. You can have perfect authentication and terrible engagement, and you'll still land in spam. You can have great engagement but no authentication, and you're vulnerable to spoofing (which tanks your reputation when spammers use your domain). You can clean your list religiously but send boring emails nobody opens, and engagement drags you down. All four have to work together.

If you're just starting to think about deliverability, check authentication first (it's the easiest to fix), then look at your list hygiene. Reputation and engagement take time to build, but authentication and hygiene are under your control right now. Want to see where you stand? Our free SPF checker will tell you if your authentication is set up correctly, and if you're stuck figuring out what to fix first, ask us (we actually pick up ;).