When a link to a Dropbox file or folder is sent by gmail, outlook.com or yahoo mail, is the content in the link read (i.e. processed) by Google or the other providers software?
I don't really know what the answer to the question is, but I do know a way to find out for yourself. This probably depends on the different providers, and the type of mails you send anyway.
You can test this very easily. Create a document on Dropbox (something random), and create a link to the document like you normally do. Then instead of just using that link, you can create a tracker link from it (eg. via bit.ly). Then you can send that mail to yourself. You can use alternate addresses you use, to make it even more realistic. After you've send the mail to yourself, you can use the bit.ly address to track how many times the link was visited. If the link was visited by somebody, and you didn't click it yourself, you can be pretty sure the link was visited by Google.
If you find this information interesting, you can also use Orangedox. This does about the same, but you don't have to manually create a tracker link (Orangedox does this for you).
I hope this way, you can find the answer for yourself.