That's the short version. The longer one is mostly about proof, because that's what a receiving server is looking for.
What the inbox is deciding
When your email arrives, the receiving server has a moment to decide whether it really came from you. It can't call you. It can only look for evidence. If the evidence is missing, it plays safe, and playing safe can mean the spam folder.
Some of that evidence is settings in your domain, and they're easy to miss.
SPF: who is allowed to send as you
SPF is a short list, published in your domain's settings, of the services allowed to send mail for it. Your email provider should be on it. So should any tool that sends for you.
What it proves: this message came from a service you approved. Without it, a receiving server has less to go on, so it may treat your mail with more suspicion.
DKIM: a signature on the message
DKIM adds a signature to every message you send, made with a key you publish in your domain's settings.
What it proves: the message was signed by your domain, and nobody changed it on the way. Without it, there's nothing on the message itself to vouch for it from your domain.
DMARC: what to do when the checks fail
DMARC is your instruction to receiving servers. It says what to do with a message that can't be tied back to your domain, and it asks them to send you summary reports.
What it gives you: a say in what happens to mail that isn't really yours, and the reports. Without it, you've told receivers nothing, and you get no reports.
The other cause: a tool blasting from your domain
The checks can all be perfect and the mail can still go to spam. The other common cause is how it's sent.
A marketing tool pushing mail to a long list from your domain. A brand-new sending address that goes straight to a big send. To a receiving server, volume with no history can look the same as spam.
That's what warming up fixes. A new sender starts small and grows, so receivers see steady, wanted mail. And logging is how you know it worked: every message recorded, so a bounce is something you can see, not something you guess at.
Which one do you have?
Check where the spam is happening before you change anything.
If mail from your own mailbox goes to spam too, start with the domain: SPF, DKIM and DMARC. If only the mail from one tool goes astray, look at how that tool sends: from your domain or its own, how much, and how fast.
If the same message lands fine at one address and in spam at another, that receiver is weighing something you can't see from here, often your history with it or that person's own filter. It isn't a setting you can change.
What we do
Authenticated sending, warmed and logged, is built once, and available to every build we do. We send through services like Resend, set up for your domain when a build sends email. Our trust page lists the services that handle it.
When you don't need us
Most of this is fixable in your domain settings in an afternoon, with your email provider's own guides, though the changes can take a while to take effect. Your provider and your domain host usually publish setup steps for each of these. If yours doesn't, ask them.
If your own mailbox is the one landing in spam and these settings are the problem, fix them yourself. You don't need us, and we'll tell you so.
If a tool is sending too much, too fast, slow it down and let it build a history. That's a setting too.
Where we come in is when sending is part of how the business runs: reminders, follow-ups, anything a customer is meant to receive without you pressing send. Software that just runs is what that looks like.