Thanks, everyone, for your help. Boy, you guys stay up late!
I don't think I explained my problem very well. If I cut and paste the Constant Contact URL into the body of the email, it works great. It's when I want to create alternate text for the link in my signature block, in this case "Click here to subscribe," then go to Edit/Link to apply the correct URL, that the extra characters are added.
Here's the entire link if you want to play around with the results:
http://visitor.constantcontact.com/manage/optin/ea?v=001WVdpo956d6k6PybRvEKkXQ%3 D%3D
And here's what Apple Mail does to it:
http://visitor.constantcontact.com/manage/optin/ea?v=001WVdpo956d6k6PybRvEKkXQ%2 53D%253D
My husband suggested a workaround for this problem: to create a redirect page on my Web site and link to that instead of directly to Constant Contact. But this Mail problem is annoying and I'd like to learn if Apple will be addressing this issue soon.