Start with the right checks.
Website and email services can use the same domain while relying on different DNS records. A working website does not prove email routing is correct.
What to do next.
Record the domain, affected addresses and the website or domain change made before the problem.
Check whether sending, receiving or both are failing. Keep a complete delivery failure message if one is available.
Ask the administrator to compare current DNS records with the email provider’s required settings. MX records direct incoming mail.
Keep existing mailboxes intact. Have the administrator check the actual provider account before changing records or recreating users.
Check the result.
Send and receive clearly labeled tests between the business mailbox and an external address. Confirm receipt in both directions.
Stop and get help if…
Nameservers changed, old records are unavailable, or the domain is managed by someone else. The website and email owners need to coordinate the repair.