A company wants to minimize the chance of its outgoing marketing emails getting flagged as...

After listing authorized email servers in DNS using SPF, the next protocol the company should apply is DKIM (DomainKeys Identified Mail). DKIM provides cryptographic assurance that email messages have not been altered in transit and that they were legitimately sent by the domain owner. Security+ SY0-701 explains that DKIM works by digitally signing outgoing emails using a private key, which recipient servers verify using a public key stored in DNS.

SPF validates where an email is sent from, but it does not protect message integrity. DKIM complements SPF by ensuring the message content is authentic, which significantly improves email reputation and reduces spam filtering issues.

DMARC (A) builds on SPF and DKIM but requires both to be in place first. DLP (B) prevents data leakage and is unrelated to email reputation. SPF (D) was already applied, as stated in the question.

Thus, the correct next step is C: DKIM.