Can I use more than five mailboxes per domain to save on domain costs when planning 25 mailboxes? Should I use the same domain for all 25 inboxes or split them?

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.