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.

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 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 three weeks.

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; their numbers are in 10 cold email infrastructure providers compared.