How mandatory is that CAA record for chatmail relays?

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Similar question is already answered here and there are more topics if you search for “CAA”:

The accounturi parameter at Chatmail server DNS records

CAA record is not very important, you can skip adding it if it’s impossible. But may be better to switch to another DNS server in any case because there is no reason for not being able to setup CAA records. If DNS provider cannot provide an interface to add CAA records they likely don’t care much about their DNS service. For the reasons to add CAA records you can read

You can remove CAA record, but it’s better to instead set up CAA records for subdomains. Subdomain CAA records override the top level CAA records: RFC 8657: Certification Authority Authorization (CAA) Record Extensions for Account URI and Automatic Certificate Management Environment (ACME) Method Binding | RFC Editor