Bind9 DNS records for SPF1 and DKIM1 for a domain used by mailgun
If you'd like to use Mailgun you will need to configure a domain or at least a hostname in a domain that will be used to send out the messages.
This will increase the likelihood of your messages arriving to their destinations and not flagged out by SPAM filters.
They have instructions for various DNS providers, but I have not seen instructions for plain old Bind9 files and it took me a while to figure them out, but now here they are:
; For Mailgun: mg 3600 IN TXT "v=spf1 include:mailgun.org ~all" mx._domainkey.mg 3600 IN TXT ("v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDKPkTXnvRQ6KIc0sA3FOZea5nunP3FhJVp6JKDkt7pqo5mtduiA/tN6E7OrE8rzgrWI50+DVYSMwxfOb5d9WLhwVlVc20dFM4ibTkcYNwDsZtPZHIbYpV/3hMjKIyZ1pauAmU05Lp0dtjf9vucIfdu4ysFol2rdenRNTt1Z+WwVwIDAQAB"); mg 3600 IN MX 10 mxa.mailgun.org. mg 3600 IN MX 10 mxb.mailgun.org. email.mg 3600 IN CNAME mailgun.org.
In this case I registered the mg hostname as in mg.hostocal.com.
Verify your configuration
There is a detailed explanation by Mailgun.
dig -t MX mg.hostlocal.com dig -t TXT mg.hostlocal.com dig -t TXT mx._domainkey.mg.hostlocal.com