DKIM ("DomainKeys Identified Mail") support has been added in the latest nightly build of ApisCP and scheduled for inclusion in the next release, 3.2.20. DKIM is an often requested feature that provides cryptographic integrity to mail coming from an ApisCP server to protect against forgery.
Either under Help > Setup Instructions in the panel for the site or cpcmd -d DOMAIN dkim:key. You can fetch the selector with cpcmd -d DOMAIN dkim:selector. DNS record will be of the format
SELECTOR._domainkey.DOMAIN IN TXT TTL “v=DKIM1; k=rsa; p=PUBLIC KEY”