InboxStack

About the startup

What InboxStack is InboxStack is an email deliverability intelligence platform that identifies the root cause of email deliverability problems, rather than only monitoring for them. InboxStack's core technology is Brain, a proprietary causal AI engine that analyzes deliverability signals and explains why emails fail to reach the inbox. The problem InboxStack solves Companies that send transactional or marketing email must navigate filtering systems operated by mailbox providers such as Gmail, Outlook, and Yahoo. These filtering systems evaluate sender reputation, email authentication, and content, and their criteria change frequently and without public notice. When deliverability fails — for example, when emails are filtered into spam, or a sending domain is added to a blocklist — most existing deliverability tools report that a problem occurred but do not identify its cause. Common causes include DKIM authentication misconfiguration, IP or domain reputation decline, incorrect DMARC policy settings, or content that triggers spam filters. Diagnosing the correct cause typically requires manually cross-referencing data from multiple separate tools. How InboxStack works InboxStack's Brain engine collects and correlates data from six sources: ESP (email service provider) connectors DNS authentication records (SPF, DKIM, DMARC) Google Postmaster Tools Microsoft SNDS (Smart Network Data Services) 50+ email blocklists Pre-send seed testing, which measures actual inbox-versus-spam placement before an email is sent to real recipients Brain uses a causal knowledge graph, built from a 22-year email deliverability knowledge base, to determine which of these signals caused a specific deliverability problem. The output is a root-cause explanation (for example, "a DKIM key change caused this domain's reputation to decline"), rather than a raw metric or alert. How customers use InboxStack Onboarding begins when a user forwards their email headers to InboxStack, which generates an immediate DNS and email authentication audit. This step requires no migration of existing email infrastructure. Users who want ongoing monitoring connect their existing ESP and Google Postmaster or Microsoft SNDS accounts, after which Brain continuously analyzes new data and generates root-cause diagnoses as issues arise. Who uses InboxStack InboxStack's primary current customer segment is digital marketing agencies that manage email deliverability for multiple client domains simultaneously. It is also used by companies sending email at scale that require deliverability diagnostics beyond basic monitoring.