How to select and copy a part of the text of an hyperlink in Mail?
I'm not sure this is what you're looking for, but when a link is displayed in Mail, if you want to select only a part of it, one trick is to put the cursor slightly to the left or to the right of the link (not too far though, or you'll select spaces too) and select the link's text from this point. 

Thus, the cursor will be in 'select' mode, and you're going to be able to select a part of the text from the link. This only works if you're wanting to select the beginning of the end of it though.