What the server does on its own

DKIM signing. Open the domain's DKIM tab in the Control Panel and click Generate key. It creates an RSA-2048 key, saves the private key, fills in the path and shows the exact TXT record with a Copy button. Once the record is published it signs every outgoing message from that domain. Ed25519 DKIM (RFC 8463) is supported alongside RSA, for shorter keys. DkimOversignHeaders over-signs the header fields you name. DkimAcceptSha1=0 refuses rsa-sha1 signatures on inbound mail.

SPF and DMARC inbound. SPF is checked as part of anti-spam scoring, with SpfVoidLookupLimit=2. DMARC policy is evaluated on inbound mail and the DMARCbis tree walk is on by default (DmarcTreeWalkEnabled=1). Aggregate reporting stays off until you set DmarcRptFromAddress.

Outbound transport security. MtaStsEnabled=1, DaneEnforcementEnabled=1 and DnssecValidationEnabled=1 are all on by default, under Settings → Security → Transport security. A recipient whose DNSSEC chain is bogus does not receive your mail rather than receiving it unencrypted. That is the point of enforcement, so read the logs before you reach for the switch.

TLS-RPT. Off until you set TlsRptFromAddress. After that the server sends daily reports to recipient domains about TLS failures it hit delivering to them.

SRS and BATV. Off by default. If you forward mail, set SRSEnabled=1 and a SRSSecret to keep forwarded mail SPF-aligned. The secret must be stable. Outstanding SRS addresses remain valid for 21 days, so changing it loses bounces for mail you forwarded recently. BATV (prvs) signs your envelope sender so you can recognise and drop backscatter. Neither ever became an RFC. Both follow the widely deployed drafts, as does everything that interoperates with them.

What you must publish yourself

PTR, and your ISP sets that one, not you. Missing PTR is the single biggest cause of spam-foldering. Then SPF, the DKIM TXT record, and DMARC. For MTA-STS on the inbound side, point mta-sts.<domain> at the server, set MtaStsHostingEnabled=1 and WebServicesHttpsPort=443, and publish the _mta-sts.<domain> TXT record. If you publish TLSA records for inbound DANE, keep AcmeReuseKey=1 so renewals do not invalidate them.

Start DMARC at p=none. Read the reports for a few weeks and confirm all your legitimate mail passes SPF and DKIM before moving to p=quarantine, then p=reject. Starting at p=reject silently loses mail from the sender you had forgotten about.

Still incomplete

ARC sealing (RFC 8617) covers mail from hosted DKIM-enabled domains. Relayed third-party mail is not sealed yet.

Chapter 8, the DNS records with example syntax: https://www.progressiverobot.com/hmailserver-documentation/#8-making-the-internet-trust-you-dns
Chapter 9.4, outbound transport security: https://www.progressiverobot.com/hmailserver-documentation/#94-outbound-transport-security
Chapter 17.6, SRS and BATV: https://www.progressiverobot.com/hmailserver-documentation/#176-srs-and-batv