Customers had started saying they never received their booking confirmations. The system showed every one as sent. They were being sent, and they were landing in spam.
The domain had no records telling the world which servers were allowed to send email on its behalf. Receiving providers, which have become considerably stricter, were treating the messages as suspicious.
Why it appeared out of nowhere
Nothing changed on the business’s side, which is what makes this so disorienting. The major mailbox providers tightened their requirements for bulk and automated senders, and mail that had been tolerated for years stopped being tolerated.
The feedback loop is also terrible. A message in spam produces no bounce and no error. The sender sees success and the recipient sees nothing, and the only signal is a customer eventually mentioning it.
The fix
Three DNS records: SPF to declare which servers may send as the domain, DKIM to sign the messages so they can be verified, and DMARC to state what should happen to anything failing those checks. Delivery recovered within a day.
This is genuinely a one-off piece of setup, not ongoing work, and it applies to every system sending as your domain: the website, the booking tool, the invoicing software, the newsletter platform. Each one needs authorising.
Worth doing today
Send yourself a confirmation from your own system, to an address on a different provider than your own, and check the spam folder rather than just the inbox.
Then list everything that sends email as your domain. Businesses routinely find four or five, some set up years ago by someone no longer there.
This is the same class of problem as a contact form that quietly stopped delivering: nothing errors, so nothing gets fixed. If confirmations of yours are going astray, describe what sends them.