Infrastructure
What is SMTP Relay?
A mail server or service that accepts outbound email and forwards it on toward the recipient's mail server.
An SMTP relay is an intermediary mail transfer agent that receives a message via SMTP and routes it onward to its destination, often handling authentication, queueing, rate control, and TLS. Relay services from an ESP or transactional provider let senders dispatch mail at scale without running their own mail infrastructure. Open relays that forward mail for anyone are abused by spammers and are widely blocklisted.
Why it matters for cold email
The relay you send through determines the sending IP and its reputation, so an abused or low-quality shared relay can land cold email in spam regardless of how good the copy is. Choosing a reputable relay with proper authentication and well-managed IPs is foundational to inbox placement.
Related terms
Stop guessing at deliverability.
Scheduler Zero handles authentication, warmup, and sender reputation so your cold email lands in the inbox, not the spam folder.