It is common practice to mail not-yet-published, unpublished and unaccessible scientific articles to colleagues working in the same field upon request. But to my experience, authors don't just mail out a copy to random strangers.
You must understand, that those articles receive reputation through the peer review process provided by respected journals. These journals are financed through sales of their issues. If authors gave away their research for free, this would cut the money from the journals and in turn destroy the peer review process that scientific reputation relies upon. So it is in their own best interest that authors restrict free copies to rare cases. There are other models for financing, and the situation is currently changing, but propriety is a strong factor when an inquiry such as yours is considered.
Now, who is a deserving recipient of a free copy?
Usually a researcher will be affiliated with some institution that has its own email domain. Usually researchers will use this email domain to send email, when they enquire with colleagues. Thus, any author can easily verify that the inquiry comes from someone actually working in their field (or at least a fellow academic).
I have sent out several requests for papers to authors. I have always explained:
- where I study/work,
- what my research interest is,
- and what I need the article for.
Sometimes I have added an explanation why I could not access the article through the usual means (no university access to the online publication) or that I needed it faster than I could receive it through interlibrary loan. Sometimes the article was not published (such as test manuals or the tests themselves), so an explanation was not necessary.
There were two authors that did not answer me.