You can send up to 30 emails per day per mailbox in total. That limit isn’t per niche; it’s 30 per day for the mailbox. With three niches you’d average about 10 emails per day per niche per mailbox, which keeps reputation safe.
There are two types of diversification we’re focusing on: mailbox diversification and domain diversification. For mailbox diversification, we take our total daily sending volume and split it across all the mailboxes. For domain diversification, we split the email volume not only across mailboxes but also across the domains those mailboxes are hosted on. Google and other providers use both the sending mailbox and the domain to determine spam score, i.e., deliverability. So the usual recommendation is no longer five mailboxes per domain; I thought it was two to three for Instantly, but realistically you can do between three to five mailboxes per domain. It’s a sliding scale of performance and deliverability versus costs. Domain costs are about $10 a year (roughly 80 cents a month). If you’re trying to save costs with 25 inboxes, you might as well save elsewhere. Mindblown Countdowns also mentions this approach.
You can buy domains directly inside Zapmail or bring your own existing ones. With ten mailboxes you’re limited to 30 emails per day total, not 30 per niche. That works out to 10 emails per day per niche per mailbox, so you won’t damage your sender reputation.
The creator recommends starting with 9‑12 mailboxes, which works out to roughly 30 emails per mailbox per day (270‑360 total). He suggests using about one domain for every three mailboxes, so 9‑12 mailboxes translates to 3‑4 domains. Domain registration costs around $5‑$10 each, and mailbox costs vary by provider—he mentions a 10% discount for Zapmail with code NYX. These are beginner guidelines from Maker School; you can increase volume if you’re not limited by resources.