Email warm-up explained: how long it takes and what it does

Warm-up is one of the most misunderstood parts of cold email. Some people skip it and burn their first domains in a week. Others treat it as magic that will fix any campaign. It is neither. Here is what it does, and how I run it.

What warm-up actually does

Inbox providers decide where an email lands partly on the sender's history. A brand-new mailbox has none. Warm-up gives it one: the mailbox sends a small number of emails each day to a network of other mailboxes, which open them, reply to some, and move them out of spam if they land there. Over a couple of weeks, the providers see a mailbox whose email is opened, answered and wanted.

That history is what lets the mailbox send campaigns later without being filtered on day one. It doesn't make bad copy good, and it can't rescue a list full of dead addresses. It just makes sure your email gets a fair hearing.

How long before campaigns start

Two weeks is the minimum I'd use for a new mailbox on a new domain. The pace mailzy uses for every new mailbox looks like this:

• Days 1 to 14: warm-up only. It starts at 5 warm-up emails a day and adds 5 a day up to 40. No campaign email at all.
• Days 15 to 21: warm-up eases to 10 a day and campaign email starts small, rising to the mailbox's full daily number over a week.
• From day 21: full campaign volume, with warm-up still running quietly in the background.

You can see the plan day by day, for your own numbers, in our warm-up schedule planner.

Why warm-up keeps running

Once campaigns start, a mailbox's reputation depends on how people react to cold email, and cold email gets fewer opens and replies than warm-up does. Keeping a small amount of warm-up going adds a steady stream of positive signals, so one bad day doesn't undo two weeks of work. It is cheap insurance.

Signs it's working

• Warm-up emails land in the inbox most of the time, not in spam or promotions.
• The mailbox's health score rises over the first two weeks and stays up.
• Nothing drops sharply when campaigns start.

Signs it isn't

If warm-up emails keep landing in spam after the first week, something else is wrong, and more warm-up won't fix it. Check, in this order:

1. The domain's SPF, DKIM and DMARC. A missing DKIM key is the most common cause. Run the domain health checker.
2. Whether the domain or its server is on a blocklist. Try the blacklist checker.
3. Whether the domain is brand new and already sending too much. Slow down.

Old mailboxes need it too

A mailbox that sent little for months looks almost new to a filter. If you're bringing an old mailbox back into use, or moving it from occasional email to daily campaigns, give it the same two weeks.

The short version

Warm up every new mailbox for two weeks before campaigns, ramp campaign volume over another week, keep a little warm-up running for as long as the mailbox sends, and if warm-up itself lands in spam, fix the setup before anything else.

In mailzy, warm-up is included for every mailbox you connect, at the pace above, with a health score and inbox and spam rates for each mailbox. See email warm-up.