Explanation
Confidentiality and integrity are two of the key facets of data security. Confidentiality ensures that sensitive information is not disclosed to unauthorized users; while integrity ensures that data is not altered by unauthorized users. These are Level 1 requirements.
Confidentiality is enforced through encryption of data at rest, encryption of data in transit, and access control.
Encryption of data in transit is accomplished by using secure protocols such as PSec, SSL, PPTP, SSH, and SCP, etc.
Integrity can be enforced through hashing, digital signatures and CRC checks on the files.
In the SRTM hierarchy, the enforcement methods would fall under the Level requirement.
References:
Gregg, Michael, and Billy Haines, CASP CompTIA Advanced Security Practitioner Study Guide, John Wiley
& Sons, Indianapolis, 2012, pp. 17-19, 20, 27-29

In this question, we have an unauthorized mail server using the IP: 192.168.2.55.
Blocking port 25 on the firewall for all unauthorized mail servers is a common and recommended security step. Port 25 should be open on the firewall to the IP addresses of the authorized email servers only (192.168.2.10 and 192.168.2.11). This will prevent unauthorized email servers sending email or receiving and relaying email.
Email servers use SMTP (Simple Mail Transfer Protocol) to send email to other email servers. Shutting down the SMTP service on the unauthorized mail server is effectively disabling the mail server functionality of the unauthorized server.
Incorrect Answers:
A: You shouldn't worry about identifying the origination point for the malicious activity on the unauthorized mail server. There isn't much you could do about the remote origination point even if you did identify it. You have an 'unauthorized' mail server. That is what you should be dealing with.
C: In this question, the email was received by the unauthorized email server (192.168.2.55) ready to be collected by the recipient. The email was not relayed (forwarded) to other email servers. Disabling open relay functionality will not stop the emails. You need to disable all email (SMTP) functionality of the unauthorized server, not just relaying.
E: STARTTLS enables TLS encryption on communications with the spam filter. It will do nothing to prevent the usage of the unauthorized email server.
References:
https://en.wikipedia.org/wiki/Simple_Mail_Transfer_Protocol
https://www.arclab.com/en/kb/email/how-to-read-and-analyze-the-email-header-fields-spf-dkim.html