My lists passed a "verified" check and still bounced 7 to 12%. Above about 5%, a new sending domain starts losing its reputation. Here's what I found and what I do now.
What I found first
My email tool's own checker gives three answers: verified, invalid, or pending. On my first sends (1 September):
301 verified leads: 0 bounces
151 pending leads: 6 of my 7 bounces
Pending is not "probably fine". Re-check once, a few seconds later. If it's still pending, don't send.
The test
I ran 45 addresses that had bounced and 45 that were delivered through Reoon, a bulk email verifier.
safe: 6 bounced, 27 delivered
catch_all: 19 bounced, 12 delivered
unknown: 10 bounced, 6 delivered
invalid: 7 bounced, 0 delivered
disabled: 3 bounced, 0 delivered
What the numbers say
Dropping only invalid and disabled barely helps.
Catch-all and unknown hold most of the bounces.
Keeping only safe brings the bounce rate to about 2% at my real base rate.
The cost: you keep roughly 56% of the list.
The rules
Run every new lead through a bulk verifier before it goes into a campaign. Budget about one credit per lead.
Send only to "safe". Hold catch-all and unknown. Never send to invalid or disabled.
Hold, don't delete. Unchecked leads go on a hold list (a blocklist) instead of being deleted, because deleting leads in most email tools can't be undone.
Check every morning. My script runs at 7:30, spends the free daily credits first, and only uses paid credits to keep about three sending days of safe leads ready. Safe leads come off hold; the rest stay held.