Diagnose in dependency order: DNS authentication, domain reputation in Postmaster Tools, list quality, sending behaviour, and only then copy. Each step only means something once the one before it passes.
1. Authentication
Run the domain check. Any red result ends the diagnosis here; fix it first. SPF over 10 lookups and missing DMARC are the two most common findings on cold email domains.
2. Reputation
Google Postmaster Tools, domain reputation tab. Low or bad means the domain is already damaged; stop cold sends, warm up for two to three weeks, and if it does not recover, retire the domain. Medium or high means the domain is fine and the problem is elsewhere.
3. List
Bounce rate over 2% tells receivers you are guessing addresses. Verify every list before sending. Catch-all domains should be sent to at lower volume or skipped.
4. Behaviour
Volume per inbox over 40 a day, new domains at full speed, no warmup, sending the same text to thousands of people at once. See the volume planner for safe numbers.
5. Copy
Last, because it is the smallest lever. Paste the email into the spam checker. Remove strong triggers, links in first touches and anything in capitals. Then stop editing copy and go back to the list.
Paste a draft and see whether it lands in the inbox or in spam, with every risky phrase highlighted.
Questions
My copy is clean but I still land in spam. Why?
Because copy is the weakest signal. Check authentication and Postmaster reputation first; those explain most spam placement.
How long until a fixed domain lands in the inbox again?
Two to four weeks of warmup with no cold sends, if reputation was low rather than bad. Bad usually does not recover.