Deliverability glossary · Updated 2026-08-10
Email warmup
Also called: Mailbox warming
Email warmup gradually increases sending from a new mailbox while generating positive engagement, so mailbox providers build a reputation for the address before it sends cold volume.
A brand-new sending domain has no history. Mailbox providers treat unknown senders conservatively, and a new address that immediately emails hundreds of strangers looks exactly like a spammer, because behaviourally it is indistinguishable from one.
Warmup fixes this by sending small volumes to inboxes that reliably open and reply, then raising the daily allowance over one to several weeks. The engagement signal — opens, replies, messages moved out of spam — is what actually builds reputation. Volume alone does not.
The weakness in most implementations is that warmup is unfalsifiable from the customer’s side. You are shown a count of threads and replies, but the receiving inboxes belong to the vendor’s network. If the inbound path breaks, the dashboard can keep reporting healthy numbers indefinitely.
The diagnostic that matters is the ratio of warmup mail sent to replies received. High sends with zero inbound is not a slow start; it is a broken loop, and no amount of additional volume will fix it.
How Raechly handles it
Warmup runs on pool accounts you register and control, so both ends of every thread are visible to you. Mailboxes graduate pending → warming → ready on a schedule, and any mailbox without verified reply capture is blocked from sending entirely.
Related terms
Sending reputation
Sending reputation is the trust score mailbox providers assign to your domain and IP based on complaint rate, bounce rate, engagement and authentication history.
Inbox placement
Inbox placement is the share of delivered messages that reach the primary inbox rather than spam or a promotions tab.
Reply capture
Reply capture is the inbound routing that delivers replies back into your sending platform.
Back to the full deliverability glossary, or read how these pieces fit together in how Raechly protects deliverability.