Some websites blanket ban email addresses that point directly to SimpleLogin MX servers (thanks Atlassian).

I found a workaround that uses an intermediate CNAME, which I'd like to leave active in case any other websites try to block it too, as it works totally fine.

Old:

MX sl.domain.tld mx1.simplelogin.co 10
MX sl.domain.tld mx2.simplelogin.co 20

New:

MX sl.domain.tld mx1.sl.domain.tld 10
MX sl.domain.tld mx2.sl.domain.tld 20
CNAME mx1.sl.domain.tld mx1.simplelogin.co
CNAME mx2.sl.domain.tld mx2.simplelogin.co

However, it seems SimpleLogin's check simply looks up the MX records.

We have detected that your domain http://sl.domain.tld/ doesn't have the DNS MX records correctly set up.

Please re-run the MX check on https://app.simplelogin.io/dashboard/domains/xxxxxx/dns and update them if needed.

Without the MX records correctly set up, emails sent to the sl.domain.tld's aliases
aren't properly and reliably handled.

Best,
SimpleLogin team.

This check would be improved if was able to follow the CNAME records to resolve the final address.

I followed the code backwards from the warning email and found this function:

class NetworkDNSClient(DNSClient):
    def get_mx_domains(self, hostname: str) -> dict[int, list[str]]:
        """
        return list of (priority, domain name) sorted by priority (lowest priority first)
        domain name ends with a "." at the end.
        """
        ret = {}
        try:
            answers = self._resolver.resolve(hostname, "MX", search=True)
            for a in answers:
                record = a.to_text()  # for ex '20 alt2.aspmx.l.google.com.'
                parts = record.split(" ")
                prio = int(parts[0])
                if prio not in ret:
                    ret[prio] = []
                ret[prio].append(parts[1])
        except Exception:
            pass
        return ret

I'd be happy to give my domain name for testing, just would prefer to not post it publicly right here.