How often are authors not allowed to send their paper by email upon request? Emailing the authors of a paper is a common way to obtain access to the paper when it is behind paywalls. However, publishing contracts may forbid  authors to send their paper by email.

Is there any research/study/survey that tried to quantify how often authors are not allowed to send their paper by email upon request? I am especially interested in papers written in English.



The RoMEO Journals database contains thousands of journals, labeled with their archiving policy (preprint/postprint/publisher's version):



Except of the database:



That gives an approximate lower bound on how often authors are allowed to send their paper by email upon request, since if an author can upload a preprint/postprint/publisher's version to a repository, they are most likely allowed to send it to someone by email upon request.



Looking at the publishers might give a decent approximation but journals from the same publisher may have different policies, e.g. http://www.nature.com/news/gates-foundation-research-can-t-be-published-in-top-journals-1.21299:


  A spokesperson for Nature’s publisher, Springer Nature, said that most Springer Nature journals do comply with the Gates Foundation policies. (Nature’s news team is editorially independent of the journal Nature.) But a “small number”, including Nature and some Nature-branded research titles, do not. 


Related: Am I allowed to share a final copy of my published paper privately?
I will speculate (and I'm guessing that a speculative answer is the best you'll be able to get to such a question) that the answer is effectively never. Specifically, I have never heard of a journal attempting to impose such a draconian restriction on authors, and moreover, I can think of two quite strong reasons why they would not do so:


Any journal that attempts to impose such a restriction is very likely to be blacklisted and boycotted by authors.
The restriction is unenforceable, since email communication is private and therefore a violation could never be detected by the publisher. In the world of dealmaking, savvy dealmakers know* that it is completely counterproductive to insert unenforceable clauses into a contract, since they will surely be ignored and their only effect will be to antagonize your counterparty and drive them to deal with a competitor instead.


* Okay, I don't actually know for a fact that "savvy dealmakers know ...", but since I know it and I'm not a very savvy dealmaker myself, I expect that it is even more obvious to savvy dealmakers than it is to me.

To summarize, if there is a journal forbidding authors from sending their paper to colleagues over email, it's a reasonably safe bet that the journal is a nonentity and can be safely ignored.