do i need 3 domains and 9 mailboxes to send 30 cold emails a day, or is that advice for people sending 3000

Going to disagree with the whole premise. If your list is 400 companies you aren't doing cold email, you're doing 400 conversations, and the infrastructure question is a distraction from the fact that you'll be through the list in thirteen working days.

Send from your normal address, plain text, no tracking pixel, no images, no sequence. The deliverability problem people solve with domain rotation is a problem created by the tools they use to avoid writing 400 emails.

The rotation advice exists because of volume, and because of what happens when a sequence tool sends the same seven paragraphs to two thousand people. At 30 handwritten a day you are a completely different animal.

Use a separate sending domain anyway - not for rotation, but so that a bad month never touches the domain your password resets go out on. One domain, one or two mailboxes, that's it. What will actually hurt you is not volume, it's complaints.

Get the authentication right and most of this stops mattering at your size. SPF, DKIM and a DMARC record on the sending domain, valid reverse DNS, TLS.

The big providers set their formal bulk-sender bar at 5,000 messages a day to their users, so at 30 a day you're not in that category - but the complaint thresholds they publish apply to everyone and are the interesting bit. Under 0.1% is the target and 0.3% is where you're in real trouble. At 30 emails a day, 0.3% is roughly one complaint every two weeks. That's how little room you have. One person hitting the button per fortnight is a failing sender.

Track replies, not opens. Open tracking adds an image, images hurt you, and the number lies anyway.

Warming a mailbox for three weeks by exchanging robot emails with other warmup accounts is mostly theatre now. What builds a sending reputation is humans replying to you. Send ten a day for the first week to the companies most likely to answer, get some replies, then ramp. Your warmup is real activity, not synthetic traffic.

Doing exactly this right now, 25 a day, second week in. Two surprises. First, putting the company's actual name and one specific detail in the opening line more than doubled my replies compared to the version where I described my product first - every guide says this and I didn't believe it until I split it fifty-fifty by hand. Second, sending from a hosted mailbox on my own domain has been fine, but I had to add the DMARC record myself because the setup wizard didn't, and I only noticed because I checked with a lookup tool. Took ten minutes.