An email that bounces costs you more than the wasted send. Every hard bounce damages your sender reputation, pushes future messages toward spam, and — at scale — can get your sending domain blocklisted.
Verification is how you avoid that. But 'verified' isn't a single thing. There are layers, each with different reliability. Here's what you actually need to know.
Layer 1: Syntax and MX checks
The cheapest checks confirm the address is well-formed and that the domain has mail exchange records. These catch typos and dead domains, but they tell you nothing about whether the mailbox itself exists.
Layer 2: The SMTP handshake
The next layer opens a real connection to the recipient's mail server and asks 'does this mailbox exist?' without actually sending a message. A clean response means the mailbox is live. A 550 response means it isn't.
This is the most useful single signal — but it's not perfect. Some servers respond '250 OK' to every address as a defensive measure. Those are catch-alls.
Layer 3: Catch-all detection
A catch-all domain accepts mail to any address @company.com — meaning the SMTP check can't tell you whether john@company.com is a real person or a typo. About 22% of business domains run catch-all setups.
For catch-all addresses, you need a confidence score: a probabilistic answer based on whether the name pattern matches known employees, whether the address has been seen in other datasets, and whether the format matches the company's documented email convention.
- High confidence: name matches a known employee + standard format
- Medium: standard format but no LinkedIn match
- Low: pattern guess only — send only if you're warmed up
How DotOutbound handles this
Every address in your prospect list is scored on a 0-100 confidence scale. We run syntax + MX + SMTP first, then route catch-alls through our pattern matcher and LinkedIn graph. We default to only sending to addresses with a confidence score of 80 or above — anything below that, you opt in deliberately.
The practical rule of thumb
Keep your bounce rate under 2% per send. Above that, mailbox providers start treating you as a spammer. Below 1% is healthy. The fastest way to get there is to never send below an 80 confidence score, especially while a sending domain is still warming up.
Lukas keeps emails out of spam folders. He's spent a decade reading SMTP logs so you don't have to.
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