Lightsail dns hosting not accepting cname

The text setup has underscores too and amazon wont accept them

According to RFC 1035, underscores are not allowed in domain names:

Characters allowed: RFC 1035 only allows letters, digits, and hyphens in domain names.

I understand you're having trouble setting up a CNAME record with an underscore in the hostname for email authentication in Amazon Lightsail's DNS hosting. This is indeed a challenging situation.

Unfortunately, Amazon Lightsail's DNS hosting does not support underscores in CNAME record hostnames. This is a limitation of the service and not something that can be easily changed.

However, there are a few potential workarounds you could consider:

1. Use a TXT record instead: Many email authentication systems (like SPF, DKIM, or DMARC) can use TXT records instead of CNAME records. TXT records in Lightsail do support underscores. Check with your email service provider if this is an option.

2. Use a different DNS provider: If you absolutely need to use a CNAME record with an underscore, you might consider using a different DNS provider that supports this feature. You can still keep your domain registered with Amazon but use another service for DNS hosting.

3. Request a change from the email service provider: While you mentioned they are unable to change it, it might be worth asking if they have any alternative authentication methods that don't require underscores in CNAME records.

4. Contact AWS Support: Although it's unlikely they can change this limitation for your specific case, they might have additional suggestions or workarounds.

Remember, when making changes to your DNS settings, it may take up to 48 hours for the changes to propagate globally. If you decide to switch DNS providers, be sure to update your domain's nameservers with your registrar.

I apologize that there isn't a more straightforward solution, but I hope these suggestions help you find a way forward with your email authentication setup.
Sources
Understanding DNS in Lightsail - Amazon Lightsail
Edit a Lightsail DNS zone - Amazon Lightsail
Troubleshoot website access errors on Lightsail | AWS re:Post