SPF
DNS TXT record listing the IPs allowed to send email on behalf of your domain.
SPF (Sender Policy Framework, RFC 7208) is a DNS record in the form "v=spf1 ip4:1.2.3.4 include:_spf.example.com -all" that tells the receiving MTA which IPs are allowed to send mail on behalf of your domain.
On every incoming message, the recipient looks at the sending IP (envelope sender), fetches your domain's SPF record and checks: is this IP allowed? If not, the email may be marked as spam or rejected.
Sendersy provisions a correct SPF record for every added domain, listing the Postal server IPs. You publish it in DNS and verify with free tools.
The main limitation of SPF is that it only validates envelope from, not header from. For full protection you also need DMARC (alignment between SPF/DKIM domains and the header From).