$ man sender-accounts

18-36 Sender Accounts ("From" Addresses)

The typical range of sending mailboxes for a cold email operation. 18 is the minimum for a meaningful campaign. 36 gives you redundancy, rotation capacity, and room to scale. Each sender account is a unique "from" address.

You can't run cold email at scale from 3 mailboxes. The volume per mailbox would be too high, the rotation wouldn't work, and one spam report tanks a third of your infrastructure. 18-36 accounts distribute risk, enable rotation, and give you the total daily volume you need. It's infrastructure planning, not overkill.

For a new partner campaign, I start with 18 sender accounts as the baseline. That gives me 18 domains and 18 mailboxes — enough for 450-540 emails/day (at 25-30/mailbox). If the contact list is over 10,000 and we need faster throughput, I scale to 36 accounts. At 36 accounts with 25/day, that's 900 emails/day. I also build in rotation capacity — if 36 is my sending set, I might provision 40-45 total so I have spares ready if a domain needs to rest.