How do shared IPs affect reputation?

You're sharing an IP address with dozens, sometimes hundreds, of other senders. That's exactly what a shared IP pool is. And because mailbox providers like Gmail and Outlook assign reputation signals to IP addresses, everyone on that address is riding the same wave, up or down.

Here's how it actually works. When your ESP routes your email through a shared IP, inbox providers look at that IP's sending history as part of their decision to deliver, defer, or filter your message. If the pool has a strong history of clean, engaged sending, your email benefits from that accumulated goodwill, even on your first send. That's the real upside of a well-managed shared IP, especially for newer senders who haven't built their own reputation yet.

The downside is the bad neighbor problem. If another sender on your shared IP starts generating spam complaints, hitting spam traps, or blasting unengaged lists, inbox providers notice. Their poor behavior drags the IP's reputation down, and your emails can start landing in spam or getting deferred, even though you didn't do anything wrong. You're collateral damage.

What actually happens when a neighbor goes rogue depends a lot on how quickly your ESP responds. Good ESPs monitor complaint rates, bounce rates, and engagement signals across their pools continuously. When a sender crosses a threshold, they get paused, suspended, or moved off the shared pool before the damage spreads too far. ESPs like Postmark are famous for being strict about who they let send, specifically to protect pool reputation. Others are more permissive, and that permissiveness has a real cost for everyone sharing those IPs.

So your deliverability on a shared IP is partly in your hands and partly in your ESP's. You control your list quality, your sending practices, and your engagement rates. Your ESP controls who else is on that pool and how fast they act when someone misbehaves.

A few things worth knowing before you settle on a shared IP setup:

• Ask your ESP how they vet new senders before allowing them onto shared pools.
• Find out whether they use segmented IP pools based on sender quality or volume, not just one giant shared pool.
• Monitor your own sending metrics closely. A sudden drop in open rates or a spike in soft bounces can be an early sign that something's wrong in the pool around you.
• Check whether your domain reputation is healthy independently of your IP. Inbox providers increasingly weight domain-level signals over IP-level ones, which gives you more control regardless of who you share an IP with.

If you want to see whether your IP is already showing signs of trouble, run it through our free blocklist checker. It takes 30 seconds and tells you if you've landed on any major blocklists.