There are several standard methods used to authenticate emails:
SPF (Sender Policy Framework) SPF allows domain owners to specify which mail servers are authorized to send emails on their behalf. By publishing an SPF record in your domain's DNS, you can help prevent forgery and spoofing.
DKIM (DomainKeys Identified Mail) DKIM adds a digital signature to your emails. This signature is verified by the recipient's mail server to ensure the email has not been altered during transit. Implementing DKIM involves adding a public key to your DNS records.
DMARC (Domain-based Message Authentication, Reporting & Conformance) DMARC builds on SPF and DKIM by providing a way for domain owners to publish policies on how to handle emails that fail authentication. It also provides reporting mechanisms to monitor the effectiveness of your email authentication.