What factors have the biggest impact on inbox placement today?

If you've ever sent a beautifully crafted email and still watched it disappear into spam, you already know that content alone doesn't determine where your message lands. So what actually does?

Here are the factors that carry the most weight today, roughly in order of impact.

1. Sender reputation (domain level first, then IP)
Your domain's sending history is the foundation. Mailbox providers track how long you've been sending, how people react to your emails, and whether your patterns look trustworthy over time. IP reputation still matters, but domain reputation has become the stronger signal. Shared IPs borrow from a pool of other senders' behavior, which is one reason dedicated IPs are worth considering once you're sending at volume.

2. Engagement history
This is where Gmail really changed the game. Gmail weighs per-recipient engagement heavily. If a specific subscriber hasn't opened your emails in six months, Gmail is more likely to route your next send to their spam folder (or the Promotions tab) than it would for a subscriber who opens every week. Other major providers have followed a similar direction. What this means practically: a small, engaged list will often outperform a large, disengaged one on placement. Every address you keep that never interacts with you is quietly working against you.

3. Authentication (SPF, DKIM, DMARC)
Authentication doesn't guarantee good placement, but missing it is a fast track to spam. Properly configured SPF, DKIM, and DMARC tell receiving servers that you are who you say you are. Without them, you're asking providers to take your word for it. Most won't. Gmail and Yahoo have made authentication a hard requirement for bulk senders since early 2024. If your records aren't in order, the other factors don't get a chance to help you.

4. Complaint rates
When someone hits "Report spam," that signal feeds back to the mailbox provider through feedback loops. A complaint rate above 0.1% at Gmail will start affecting your placement. Above 0.3% and you're in serious trouble. High complaints tell providers that your recipients didn't want your email, which is the clearest negative signal you can generate.

5. Content quality
Content still matters, but it's the last factor on this list for a reason. A sender with strong reputation and great engagement can get a mediocre email through. A sender with weak reputation won't be saved by beautiful copy. That said, certain content patterns (heavy image-to-text imbalance, trigger words in subject lines, broken HTML) can still tip a borderline email into spam. Don't ignore content, just don't expect it to compensate for the four factors above.

The shift that's happened over the past few years is real. Providers have moved from asking "Does this email look okay?" to asking "Does this recipient want this email?" Reputation and engagement answer that second question far better than any content check does.

Not sure how your current setup scores? You can check your authentication records with our free SPF checker or DKIM lookup. If you want to talk through what to prioritize first, our SOS hotline is free and we actually pick up.