Answer:
a. Open relay agent.
Step-by-step explanation:
Open relay agent is a protocol of internet configured in such a way that it allows anyone could send emails, this was the initial configuration on the internet. But this protocol is unpopular because of the spammers used for sending a lot of spam.
After a lot of abuse by spammers, operating an open relay came to be frowned upon among the majority of Internet server administrators users.