Ah, I think I have it figured out....
You must be using an older version of the plugin. The older versions used to conflict with Joomla's email protection plugin. The email protection plugin would scramble email addresses in such a way that spambots could not read the email.
To fix this try this:
1. Go to Administrator>>Extensions>>Plugin Manager.
2. Filter the list by "System" (use the drop down menu in the upper left of the list).
3. Find System - Nice PayPal Button and change its order number, so that it will be the last in the list.
4. Then click the little icon at the top of the order column to submit your order numbering.
This should resolve the issue as the email protection plugin will run before the Nice PayPal Button plugin. In this way, the email address in the PayPal Button will not get scrambled.
~ All the best