Final answer:
From the given options, a proxy server would be closest to acting like a 'post office' for Internet hosts, but it does not perfectly fit the role of managing emails. The correct term for such a server is a Mail Transfer Agent (MTA) or mail server.
Step-by-step explanation:
The specialized server that acts as a post office for a particular Internet host, managing the sending and receiving of emails, is a Mail Transfer Agent (MTA), often referred to simply as a mail server or email server. However, from the options provided, the closest answer would be a proxy server (c), which performs functions on behalf of other computers, like a mail server does for email. However, the question seems a bit misleading as none of the options perfectly fit the description of an email server. Proxy servers typically are intermediaries for requests from clients seeking resources from other servers. A DNS server translates domain names into IP addresses, a router directs traffic on a network, and a modem connects to the internet. None of these is specifically designed to act like a post office for email.