My reply rate dropped from 3.8 to 0.4 percent overnight. Here is the 5 day diagnostic that fixed it
spam_diag_ryan · 2026-06-18 · 1,640 views
Three weeks into a new campaign and my reply rate dropped from 3.8% to 0.4% in 48 hours. No copy changes. No sending volume changes. Just a cliff.
Here is the exact diagnostic process I ran and what I found.
Day 1: Ran a GlockApps test.
Sent test emails from all 12 inboxes across my domains. Results: 3 domains at 90%+ inbox placement, 4 domains at 40 to 60% placement, 2 domains landing almost entirely in spam. That told me immediately it wasn't a copy problem. It was domain-specific.
Day 2: Checked blacklists and DNS.
MXToolbox showed two of my worst-performing domains were listed on Spamhaus. I had no idea. The listing was probably triggered by a batch of bad contacts I hadn't cleaned properly before sending. That list had not gone through ZeroBounce before I loaded it. Lesson burned into my brain permanently.
Day 3: Fixed the list hygiene problem first.
Pulled the entire contact database through ZeroBounce. Found 11% of emails were either invalid or catch-all risky. Removed all of them. I should have done this before sending day one.
Day 4: Submitted Spamhaus delisting requests.
For both flagged domains, I submitted delisting requests through Spamhaus's lookup tool. They approved both within 24 hours after I demonstrated the lists were cleaned and bounce rates had dropped. You have to show the problem is fixed, not just ask nicely.
Day 5: Reran GlockApps.
Inbox placement back above 88% across all domains. Started sending again with the clean list only.
What I do differently now.
Every list through ZeroBounce before it touches Smartlead or Instantly. Weekly GlockApps checks across all domains, not just when I notice a problem. PuzzleInbox handles my DNS so SPF, DKIM, and DMARC are always correctly configured. The deliverability drop was a list hygiene problem, not an infrastructure problem. The infrastructure actually helped me recover faster.
Deliverability problems almost always come from one of three things: dirty lists, DNS misconfiguration, or sending volume that exceeds what the inbox reputation supports. Diagnose in that order before you blame anything else.