# One client's bad list just put every client's domain at risk.

> If you send on behalf of multiple clients, a bounce spike or spoofing problem on one domain doesn't always stay contained to that one domain - shared sending infrastructure and shared reputation can spread the damage further than the client who caused it. Isolating and monitoring each domain separately is the fix, not catching problems after a client complains.

## Why one client's problem becomes everyone's problem

Two things make this spread: a shared sending domain or IP pool, where one client's poor list hygiene drags down a reputation other clients rely on too, and simply not knowing which client's domain is the one causing a problem until an overall number looks bad. Isolating sending per client, and checking each domain's own posture rather than an aggregate, is what keeps one bad list from becoming every client's deliverability issue.

## Check this before you take on a new client's sending

Before you send a single campaign on a new client's behalf: verify their existing list (a purchased or long-unused list is the single most common cause of an immediate bounce spike), and check their domain's current SPF, DKIM and DMARC posture rather than assuming it's set up correctly. A domain with no authentication at all is a liability the moment you start sending real volume through it.

## Keep checking, not just once

A client's domain posture and list quality aren't fixed facts - a client might add a new tool that sends as their domain without telling you, or a once-clean list might go stale over months of no sending. Re-check each client's domain on a schedule, and verify lists again before any campaign that's been sitting unused for a while.

## Questions

**Can I check multiple client domains from one account?**
Yes - verify and monitor SPF, DKIM and DMARC for every domain you manage, not just one, from the same account.

**What's the first thing to check for a new client?**
Their domain's current DMARC/SPF/DKIM posture and the health of whatever list they want to send to first - both before the first real campaign goes out, not after.

**Does one client's bounce rate affect another client's deliverability?**
It can, if they share sending infrastructure or a reputation pool. Isolating sending per client and monitoring each domain's own numbers separately is what prevents that.

Tool: [Check a client's domain posture](https://www.emcheq.com/mail-security)