The Supreme Court of Canada in Crookes v Newton held that the mere linking to a web site that contained defamatory material did not make the linker liable for defamation. Adding content to the link might change that result.
The Supreme Court of British Columbia has recently held, however, that offering a link to an email program (e.g. ‘mailto:’) on a web page that contains defamatory material constitutes republication of that material, apparently whether or not anyone used it.
Weaver v Corcoran 2015 BCSC 165 (CanLII)
Here is the main passage on that point:
 The invitation to email the article to a friend meets the test set out in the jurisprudence that re-publication is the “natural and probable result of the original publication”. The invitation is to email an article, the content of which is known and indeed created by the defendants. This is unlike the situation in Crookes which involved a website concerning commentary on various issues with hyperlinks to other websites, the content of which is not controlled by the operator of the website. In this context, I am prepared to conclude that a reasonable inference could be drawn of publication to a third person of each of the articles at issue.
(You will see from that paragraph number that it’s a long decision.)
Most web browsers offer the capacity to send on either a link to the page being viewed or the page itself. Does that make the browser liable, under this ruling, or the publisher of the original page more deeply liable because it must be taken as general knowledge that anything on the web can be forwarded (not just linked to) through the browser used to view it? Or must there be an ‘invitation’? If so, does it make sense to treat what is probably a pretty routine feature of web pages of this sort (newspaper pages, but also magazines and others) as an automatic increase in aggravation of defamation?