Email mailbox warmup is the process of building a new email account's sender reputation before using it for cold email outreach. A brand-new mailbox — whether on Google Workspace or Microsoft 365 — has no sending history. Inbox providers treat new senders with maximum suspicion. Warmup simulates normal email activity over 3 to 4 weeks, establishing the account as a legitimate sender before any cold email is sent from it.
How Warmup Works
Warmup tools like Instantly operate a network of real email accounts that exchange emails with each other, simulating human-like sending behavior. A new mailbox sends a small number of emails per day to the warmup pool, receives responses, and generates positive engagement signals (opens, replies, mark as important). Volume gradually increases each day. After 3 to 4 weeks of consistent warmup, the mailbox has built enough sending history to handle cold email volume without triggering spam filters.
How Long Warmup Actually Takes
Minimum viable warmup takes 14 days for a Google Workspace mailbox, though 21 to 28 days is the professional standard for domains that will see sustained high-volume sending. Microsoft 365 mailboxes typically require 3 to 4 weeks due to stricter reputation requirements from Outlook's filtering systems. Rushing warmup — sending cold emails before the warmup period ends — is one of the most common reasons cold email campaigns fail on launch.
Warmup After Launch
Warmup should never fully stop. Even active cold email mailboxes benefit from running warmup in the background throughout their lifespan. Warmup signals dilute the reputation cost of unavoidable spam complaints and occasional bounce rate spikes. Instantly allows you to run warmup simultaneously with live cold email sending. Omni keeps warmup running on every active mailbox in its client campaigns as part of the fully managed outbound system.