How Many Mailboxes Should I Put on One Cold Email Domain?

The number

Up to 5 mailboxes per domain. With up to 25 cold emails per mailbox a day, that caps one domain at 125 cold emails a day, excluding warmup.

Mailboxes and domains for one campaign

Divide the cold emails you want to send a day by 25 to get mailboxes, then divide the mailboxes by 5 to get domains. 1,000 cold emails a day needs 40 mailboxes on 8 domains. New mailboxes send the full 25 from day 14, after 14 days of warmup. 40 mailboxes cost $100 a month.

Why the domain matters

Receiving providers don't only look at the mailbox. Google's sender guidelines say Gmail "tracks volume, feedback, and limits per domain and IP address," and that "DKIM and SPF quotas are specific to your domain." Spam reports work the same way: "If messages from your domain are frequently reported as spam, future messages from you are more likely to be marked as spam. Over time, user spam reports can lower your domain's reputation."

That has two consequences:

• Mailboxes on one domain share one reputation. A single mailbox on a domain has little reputation of its own. If the domain gets a bad name, all its mailboxes go with it.

• More mailboxes on one domain means more volume on one reputation. Ten mailboxes on one domain is the same as doubling that domain's daily volume.

What 5 per domain looks like

Fewer mailboxes per domain spreads the risk further but costs more domains. Five is the balance we recommend for most senders.

What our sending data shows

On mail to Google recipients, reply rates fall as each mailbox sends more: our fitted trend over 19 weeks gives 1.01% human replies at 29 emails per mailbox a week and 0.21% at 923. That matters for most programs, because in the median cold email program we see, 46% of recipients are on Google and 20% on Microsoft 365 or Outlook.com.

Our data does not isolate the number of mailboxes per domain; it shows what load per mailbox does, and it is a correlation, since heavy senders also differ in lists and copy. The 5-per-domain cap keeps each mailbox at 25 a day and each domain at 125. To plan mailboxes and domains for a target volume, use the cold email capacity calculator.

How we measured. Replies: weekly cold sends and human replies to Google-hosted recipients from about 25 workspaces, January to June 2026, with a log-log trend fitted across the weekly points. Recipient mix: median share across the 29 workspaces that sent more than 1,000 cold emails through our relay from October 8 to 15, 2026, with the provider read from each recipient domain's MX record. Outreach2day data, October 2026.

What doesn't give a separate reputation

• Aliases. An extra address or alias on the same mailbox or domain shares its reputation. It does not add capacity.

• Subdomains of your main domain. Google counts subdomains toward the primary domain for bulk sender status: its sender FAQ says "Messages sent from the same primary domain count toward the 5,000 limit." Use separate domains. Subdomains of your company domain count toward the same limit. See using your main domain.

Watch each domain as well as each mailbox

Even at 5 mailboxes, a domain can wear out if it sends the same copy for weeks. Track results per domain:

1. If one domain drops while other domains on the same sending IPs are fine, that domain is the problem. Try a fresh campaign or list on it before you replace it.

2. If all domains drop together, look at the copy, the sending IPs or your total volume instead. See burned domain.

Keep a few spare domains warmed up so you can replace a domain without waiting two weeks.

Questions

How many cold emails should one mailbox send per day?

Up to 25, excluding warmup. New mailboxes send the full 25 from day 14, after 14 days of warmup. To send more, add mailboxes and domains and keep each mailbox at 25; the reasons are in cold emails per mailbox per day.

How many cold emails can I send per day from one domain?

Up to 125 cold emails a day: up to 5 mailboxes at up to 25 each. Warmup emails don't count toward it. Gmail tracks volume per domain as well as per mailbox, so more mailboxes on one domain only add volume to the same reputation.

What we recommend at Outreach2day

Up to 5 mailboxes per domain, up to 25 cold emails per mailbox a day, and warmup on every mailbox. For 1,000 emails a day that is 40 mailboxes on 8 domains. The send 100,000 cold emails a month guide covers sizing for larger volumes, and the cold email cost calculator counts domains for any number of mailboxes per domain and prices the setup. Other providers recommend 2 to 3 mailboxes per domain; the providers are compared in cold email infrastructure providers compared.