We are running 15.7 and this is a CONSTANT problem for us. I've been trying to get everyone off MAC mail, (because mail sent from MAC doesn't always appear in the smartermail interface and we also end up with 3 sent messages boxes) however, you cannot send a link copied from the browser window through Smartermail without having it break. The link below we copied and pasted into an email in the Smartermail interface, it broke. Copied and pasted the same link into MAC mail, it delivered intact.
So, it is not the link, not user error. This is a glitch of some kind.
While the above represents what the link looks like in Smartermail, (I couldn's paste the actual text in here because hyperlinks are not allowed in posts) in the email the top line is a live link that can't complete properly and will give you a 404 message. The bottom line is not a link. The only way to use this link is to copy and paste it into the browser, removing the space at the break point. Not something staff here is interested in doing.