From https://en.wikipedia.org/wiki/CNAME_record: "If a CNAME record is present at a node, no other data should be present; this ensures that the data for a canonical name and its aliases cannot be different."
So if you're looking up the MX record for domain, but happen to find a cname for domain to www.domain , it will follow that and won't find any MX records for www.domain.
The correct approach is to create a cname record from www.domain to domain, and have the A record (and MX and other records) on the apex.
Most DNS providers have a proprietary workaround to create dns-redirects on the apex (such as AWS Route53 Alias records) and serve them as A records, but those rarely play nice with external resources.
