You don't know if you're landing in spam

The most uncomfortable realization from two years of cold email: no mail provider tells you that your message landed in spam. No dashboard, no “delivered” checkmark, no bounce notification. The email simply vanishes silently into a folder nobody opens — and your reply rate drops without you knowing why.

So we built an automatic deliverability monitor. These are the field notes — no theory-speak, just what actually makes the difference.

1. The warmup score fooled me the longest

98 out of 100. Green checkmark. And yet zero replies came from the Microsoft segment. The reason: the warmup score only measures placement within the warmup network — accounts that write to each other. It has nothing to do with your real target audience. It’s the most dangerous vanity metric in cold email, because it lulls you into a false sense of security while the domain is dying.

2. The biggest lever isn’t copy, it’s routing

Gmail senders to Gmail recipients. Microsoft to Microsoft. This ESP matching moved the needle for us more than any subject-line optimization. But: it only works if both sides exist. Whoever has a pure Google sender fleet can’t “match” Microsoft recipients — there’s nothing to route to. And if you add Microsoft mailboxes: connect via native OAuth, not generic IMAP, otherwise they’ll never be recognized as Microsoft.

3. Aggregated numbers lie by omission

Our overall bounce rate looked healthy for months. Until we split it by recipient provider: Gmail flawless, Microsoft collapsing. In DACH B2B, the Microsoft share is high — whoever doesn’t split by ESP doesn’t see half the market. Microsoft is its own, harder game: stricter filters, lower limits, slowest recovery. Treat it as its own track.

4. Speed where it’s safe — hands on the wheel where it isn’t

A Monday problem is a burned domain by Friday if a human has to notice it first. So we automate the safe side — alert, throttle, pause on clear thresholds — and gate the destructive one: swapping a burning domain for a healing domain is a decision a human makes. The same human-in-the-loop principle that runs through our whole stack.

The short version

Scale through more domains and mailboxes, never through more volume per box. Measure bounce and reply separately by recipient provider. Classify the provider from the MX records, not from the tool’s label. And treat every “95% inbox guarantee” as marketing, not as a metric.

The full monitoring mechanics — threshold table, provider limits, the 10-point checklist — are laid out in detail here: Monitoring cold email deliverability.

CegTec builds and operates signal-based outbound for the DACH region — including the deliverability monitoring behind these field notes.