NOTE: This is the second blog in a series on email deliverability. Click here to read the first article.

Every mailbox provider keeps a running assessment of every sender who mails their users. That assessment is your sender reputation, and it’s one of the biggest factors determining whether your email lands in the inbox or gets quietly redirected somewhere else.

Reputation isn’t a badge you earn once and keep forever. It’s dynamic, shaped by your sending behavior and recalculated with essentially every email you send. A strong reputation built over months can slip after a single poorly planned send. A shaky one may bounce back after a few weeks of clean, consistent sending.

What Builds Sender Reputation (and What Quietly Wrecks It)

Sender reputation is built over time, through signals that are easy to overlook. Some strengthen your standing with mailbox providers while others can quietly chip away at it. Let’s look at a few.

  • Sending volume: Mailbox providers get nervous about sudden spikes in the number of people you’re sending mail to. If you normally send emails to 5,000 people and one week you blast 50,000, that looks a lot like spam behavior, even if it’s completely legitimate. Aim for a steady, predictable sending pattern, and ramp up volume gradually when you’re adding a new IP or domain into the mix.
  • Bounces: A soft bounce is a temporary hiccup, like a full inbox or a server that’s briefly down. A hard bounce means the address doesn’t exist at all, and it should never be mailed again. Repeatedly sending to addresses that hard bounce is one of the fastest ways to damage your reputation because it signals sloppy list practices. Most email service providers will auto-suppress these for you, but it’s worth confirming that setting is actually on.
  • Engagement: When someone marks your email as spam, that’s a direct signal to the mailbox provider that you shouldn’t be trusted. When someone rescues your email from spam, replies or moves it into a folder, that reads as a vote of confidence. Multiply either of those across your whole list and you can see how your sender reputation can move quickly after each send.
  • Shared vs. Dedicated IP: On a shared IP, you’re pooled with other senders, which means their sloppy habits can drag your reputation down. That’s true even if your own program is spotless. A dedicated IP puts the reputation entirely in your hands, for better or worse. Plenty of programs start on a shared IP and move to a dedicated one once they’ve got enough steady volume to keep it “warm.”

Keeping Score, Literally

Reputation is often expressed as a score on a scale from 0 to 100. As a general guideline, anything below 85 is worth paying attention to. The strongest, most established sending programs typically sit somewhere between 94 and 98. It’s not a perfect predictor of inbox placement (no single score is), but it’s a useful early-warning system. Marketing partners like Simantel can help you check where your program stands and build a plan to help you maintain the best sender reputation possible.

Blocklists: Reputation’s Public Record

A blocklist is a running list of IP addresses or domains that have been flagged for spam-like behavior. Some are run internally by mailbox providers themselves, Gmail keeps its own, for instance. Others are maintained by third parties and referenced by mailboxes around the world.

Landing on one isn’t necessarily a five-alarm fire. Depending on the list and how it’s used, it might only affect your delivery to a specific mailbox provider or a specific region, rather than tanking your deliverability everywhere at once. Still, it’s worth checking periodically. Most blocklist providers have their own process for getting your IP address or domain removed once you’ve cleaned up whatever triggered the listing.

Don’t Skip List Validation

A lot of email programs never run their list through a validation service. This will show up in bounce and complaint numbers long before anyone traces it back to the list itself. Validating your list catches invalid, risky and outdated addresses before they ever touch your sending reputation. It’s one of the highest-leverage things you can do for a program, and it doesn’t take long to set up. If you’re not sure where to start, Simantel can help you put a list validation process in place.

The Habit That Protects All of This

None of this requires constant vigilance. It requires consistency: send on a predictable cadence, keep an eye on your bounce and complaint numbers, and don’t let your list get stale. Reputation rewards boring, steady behavior far more than it rewards a perfect single campaign.

Next in this series: SPF, DKIM and DMARC, the technical handshake that proves an email is really coming from you.

Not sure where your sending reputation actually stands? Download our Email Deliverability Checklist to see how your program stacks up or set up a consultation with our team to check it out together and build a sending cadence that protects your reputation. Let’s talk.