Managed DKIM/SPF services and Postal DNS Checker

## Describe the bug

Not sure a bug, but maybe needs extra code to check DKIM and SPF for managed services.
I use RedSift OnDMARC services to manage my DKIM, DMARC and SPF records with Dynamic services.
In this example, my SPF record is **"v=spf1 include:_u.DOMAINNAME._spf.smart.ondmarc.com ~all"** and all the "Include" mechanisms are fed from that SPF record from OnDMARC.
Same with DKIM. My DNS record for my domain is **_domainkey.DOMAINNAME | NS |  ns-dkim.ondmarc.com** and all my DKIM entries are managed in OnDMARC portal e.g. - **postal-ditccq | TXT | v=DKIM1; t=s; h=sha256; p=MIGfMA0GCS.....**
I get **Warning An SPF record exists but it doesn't include spf.mailer.POSTALSERVER.co.uk** and **Warning No TXT records were returned for postal-dITCcQ._domainkey.DOMAIN**

Running a DKIM checker on MXToolbox reveals he correct DKIM key - **DOMAIN:postal-dITCcQ**

## Expected behaviour

It'd be clever if the Postal DNS checker followed SPF and DKIM records to check for existence of the real records