Delivered is not the same as inboxed
A message that reaches the receiving server has been delivered. Whether it lands in the inbox or the spam folder is a separate decision the provider makes afterwards, based largely on how the sending domain has behaved.
Most reporting shows the first number, which is why teams can watch a "98% delivered" dashboard while their reply rate collapses.
What actually moves the judgement
Bounce rate is the strongest signal. Mail to addresses that do not exist implies a sender who does not know their recipients, which is the defining characteristic of spam.
Complaint rate — recipients marking messages as junk — is weighted heavily and recovers slowly.
Volume shape matters independently of content. A domain that sends forty messages a day for a year and then sends four thousand looks compromised, because that is what a compromised domain looks like.
The cost nobody attributes correctly
Reputation attaches to the domain, not the campaign. Damage done by prospecting affects invoices, contracts, support replies, and password resets from the same domain — and it surfaces weeks later, when nobody connects the customer who never received their invoice to the sequence that ran last month.
Verification, and the case it cannot resolve
Verification protects reputation by confirming an address exists before anything is sent. It works in escalating tiers: structural validation, then a probe asking the receiving server whether the mailbox exists.
The case neither tier resolves is the catch-all domain — one configured to accept mail for any name at it, whether or not that mailbox exists. The server says yes to everything, so the probe proves nothing. Large enterprises are disproportionately configured this way, which means the accounts most worth reaching are exactly the ones verification struggles with.
Treating catch-all as valid reintroduces bounces. Treating it as invalid discards your best accounts. The workable answer is a probabilistic score with a threshold, and a route to a different channel for anything below it.
Never guess an address pattern
Inferring first.last@company.com from a name is the single most common cause of avoidable bounces. It is free to do and the cost is deferred, which is exactly why the habit survives. An address should come from a source that verified it, or the lead should be reached another way.