Deliverability is now a structural concern, not a tactical one.
1 in 6 marketing emails failed to reach the inbox in 2024 after Gmail and Yahoo tightened enforcement. For any organisation spending serious budget on outbound or demand generation, that stat should be front of mind.
The 5-layer system
I think of deliverability as a five-layer system. Problems can originate at any layer, and most teams troubleshoot at the wrong one.
Layer 1: Domain reputation. Your domain has a reputation score with email providers. If it’s been flagged or damaged, everything you send from it gets penalised. This is the layer most teams ignore until it’s too late.
Layer 2: Email tenant. Whether you’re on Google Workspace or Microsoft 365, the tenant itself has a reputation. A new tenant with no sending history behaves differently to an established one.
Layer 3: Individual inbox health. Each inbox you send from has its own reputation. If one inbox in your tenant is burning through contacts with low engagement, it can drag down the others.
Layer 4: Message content. The actual email. Spammy language, too many links, heavy HTML formatting, identical copy sent at volume. All of these trigger filters at the content level.
Layer 5: The recipient’s side. The recipient’s email provider, their own spam settings, their engagement history with similar messages. You can’t control this layer, but you should know it exists.
Most teams troubleshoot at Layer 4 (rewriting the email) when the real issue is at Layer 1 or Layer 2 (domain or infrastructure). You can write the best email in the world, but if your domain reputation is damaged, nobody is going to see it.
Don’t send cold email from your primary domain
This is something I feel strongly about. Your primary domain is a business asset. If it gets flagged, your customers don’t receive invoices, support threads, contracts.
You may have got away with it so far. But who knows how Google and Microsoft are going to penalise outbound messages in the future. It’s worth considering the risk now rather than finding out the hard way later.
The infrastructure split most teams get wrong
There’s an important distinction that a lot of teams blur.
Your newsletter and nurture sequences run through a dedicated platform off your primary domain to people who opted in. Your cold outbound runs from disposable secondary domains to people who’ve never heard of you.
These are completely different systems with completely different rules. Running cold outbound through your marketing automation platform off your primary domain isn’t a deliverability strategy. It’s a way to eventually break your newsletter too.
Keep them separate. Different domains, different infrastructure, different rules.
A few data points worth knowing
Gmail inboxes get roughly 2x the open rate of Outlook. 74% of first replies arrive within the first hour of a campaign launching. And Sunday is surprisingly as effective as Monday for send timing.
None of these will matter if the infrastructure isn’t right. But once it is, they’re useful for optimising.