What's "垃圾邮件"in English, spam or junk mail?
If you're talking about unwanted letters to our email inbox, use spam.
Junk mail refers to the traditional advertising letters delivered by the postmen to our home or office.
Spam is strictly for the Internet, even though its content is every bit as trashy and rubbishy as traditional junk mails - and considering its prevalence, proving a headache to some electronic mail users as well as service providers.
My email boxes, for example, used to be inundated with spam before the servers apparently took extra preventive measures. I'm talking about getting dozens of spam letters over, say, a weekend. They tout services ranging from - well, you know what I'm talking about, you are probably getting the same things, too - lottery tickets, mortgage loans, questionnaires, weight loss schemes, to Viagra pills and Girls Gone Wild DVDs….
Back to the question - spam or junk?
If there's no need for specification, junk mail will do for all garbage in the mail. In actual conversation, you might want to be more specific. In the following example, from a BBC Learning English article on Stealth Marketing sent by a reader via email, many marketing tricks including junk mail and spam are each distinguished from one another:
"How do you decide what particular product to buy? These days we are bombarded by advertising and marketing messages: on television, on radio, product placement in films, adverts on the underground, posters in the streets, people handing out fliers, pop ups when we browse the internet, junk mail sent to our homes or spam to our email, cards left under the windscreen wipers of our cars, it goes on and on..."