What factors determine inbox vs spam?

And every Every time you hit send, the receiving mail server runs your message through a scoring process that pulls from several different signals at once. It's not just one thing that gets you flagged. It's the combination.

But Here's what actually goes into that decision:

• Sender reputation is the biggest lever. Mailbox providers track your history at both the IP level and the domain level. If you've been sending clean mail for years, that history works in your favor. If you've had a rough few months, it follows you.
• Authentication results tell the filter whether you are who you claim to be. SPF, DKIM, and DMARC all need to pass and align. Missing or broken authentication is a fast track to suspicion, regardless of how good your content is.
• Engagement signals are increasingly important. If your last 10,000 emails got deleted without being opened, that tells the filter something. Opens, clicks, replies, and saves all count the other way.
• Complaint rates hit hard. When recipients mark your mail as spam, that vote registers directly with the mailbox provider. Even a small spike in complaints can shift your placement quickly.
• Content analysis still matters, but it's less decisive than it used to be. Spam-heavy phrasing, suspicious links, and known bad patterns all add to your score. (Of course, no one publishes the exact list of triggers.)
• Per-user history is the layer most people forget about. Gmail and Yahoo Mail both personalize filtering. If a specific recipient has never opened your emails, your messages to them face a harder path than to someone who clicks every week.

These signals don't work in isolation. They feed into machine learning models that weigh them differently depending on the mailbox provider and the individual recipient. There's no single passing grade. A strong reputation can cover a weak content score. A high complaint rate can cancel out perfect authentication.

The practical takeaway is that you can't optimize just one factor and expect consistent inbox placement. You need a decent baseline across all of them. Most senders who are struggling aren't failing everything. They have one or two weak spots dragging the whole score down. The hard part is knowing which ones.

So if If you want to start diagnosing, check your sender reputation and authentication setup first. Those two are the most fixable and the most commonly broken. Our free blocklist checker and SPF checker are a good starting point, or just ask us directly if you're not sure where to look.