Email and Second class

Email noun - (computer science) a system of world-wide electronic communication in which a computer user can compose a message at one terminal that can be regenerated at the recipient's terminal when the recipient logs in.

Second class and email are semantically related. In some cases you can use "Second class" instead a noun "Email".

Second class noun - Not the highest quality in a classification.

Email and second class are semantically related. Sometimes you can use "Email" instead a noun phrase "Second class".