Infrastructure is not the whole story
Deliverability depends on three things: infrastructure, content, and sending behavior. TenantCore addresses infrastructure. Content quality and sending behavior are outside its scope but infrastructure sets the ceiling that the other two factors work within. Bad infrastructure guarantees poor deliverability. Good infrastructure does not guarantee good deliverability but it removes the most common reason cold email ends up in spam.What receiving servers evaluate
When an email arrives at Gmail, Outlook, or any other receiving mail server, the server evaluates: Authentication — Does the sending domain have valid SPF, DKIM, and DMARC records? Does the message pass those checks? Sender reputation — Does the sending IP address or domain have a history of spam? How old is the domain? How much volume does it typically send? Engagement signals — Do recipients open, reply, and mark as not spam? Or do they delete, ignore, and mark as spam? Content signals — Does the message contain spam trigger words, suspicious links, or unusual formatting? TenantCore directly improves the first two. Authentication is handled through proper DNS setup. Sender reputation is protected through volume limits, domain isolation, and the 3-mailbox-per-domain ceiling.Microsoft 365 and deliverability
Microsoft 365 is a premium sending infrastructure. Outbound mail routes through Microsoft’s IP ranges, which carry strong baseline reputations with major receiving servers. This is one of the primary reasons operators choose Microsoft over generic SMTP providers for cold email. Microsoft’s IP ranges are among the most trusted on the internet. Receiving servers at Gmail and other major providers have decades of signal associating Microsoft’s infrastructure with legitimate business communication. That baseline trust is something a generic SMTP provider or a new dedicated IP simply cannot replicate. When you send through Microsoft 365, you are starting from a position of credibility that most other sending infrastructures cannot offer.Common infrastructure mistakes
Too many mailboxes per domain
The more mailboxes you stack on a single domain, the more total volume signal that domain produces. Receiving servers evaluate domain-level signals, complaint rates, bounce rates, engagement patterns, and volume relative to the domain’s age and history. A new domain suddenly producing high volume across many mailboxes accumulates negative signals faster than one sending conservatively across fewer mailboxes. Running large numbers of mailboxes (e.g. 50–100+) on a single domain also creates a single point of failure. Email providers like Google and Microsoft track reputation primarily at the domain level, so if that domain is flagged, throttled, or blacklisted, every mailbox on it is affected. The 3 mailbox per domain ceiling keeps per-domain volume proportional to legitimate business usage, distributes risk across domains, and ensures each domain can build and maintain a clean sending reputation over time.Sending from primary domains
Your company’s primary domain (acme.com) carries your brand reputation. Sending cold email from it exposes that reputation to spam complaints and blacklisting. Use dedicated sending domains that are related to your brand but not your primary domain (tryacme.com, acmehq.com, etc.).
Skipping the warm-up period
New domains have no sending history. Receiving servers are conservative about accepting mail from domains they’ve never seen before. Ramping too fast, starting at 100 emails per day on day one, is a reliable way to trigger spam filters. Start low, increase gradually, and monitor your metrics.Missing or misconfigured DNS records
SPF, DKIM, and DMARC failures cause immediate deliverability problems. Mail that fails authentication is either rejected outright or routed directly to spam. TenantCore generates the correct records for each domain — add them to your registrar before sending.Monitoring deliverability
TenantCore does not provide inbox placement testing or spam score analysis. For ongoing deliverability monitoring, use dedicated tools:- GlockApps — inbox placement testing across major providers
- MXToolbox — blacklist monitoring and DNS record validation
- Mail-Tester — quick spam score check for individual messages