It is a best practice when linking to any site other than your own, to make it open in a new window so as not to lose the reader. Since this is an email cms, that seems like it would have automatically have been the design.
I avoid inserting any links that aren't absolutely necessary because I don't want to lose email readers.
We've identified a specific issue with clickable buttons in the permalink and View As Webpage link. When viewing these pages, clicking a button will cause the new page to open in the same tab instead of in a different tab which is the behavior of emails accessed through an email client. Text links are not affected by this issue. We submitted a request to our development team to have the URLs behave uniformly or to have a choice between links opening in a new tab or window. There are no immediate developments happening but we will continue to keep this thread updated with any movement.
If you are seeing buttons or text links acting differently outside of the behaviors listed above, please collect the following information:
if the link was inserted as a button or text
which button or text link is acting this way
You can then email us communitysupport(at)constantcontact(dot)com for further assistance.