When you right-click on a mailto link and left-click on "Copy email address", a decoded, comma-separated list of email addresses from the MAILTO keyword and all TO keywords should be copied to the clipboard.
Remember, mailto:?to=email%40site.com is equivalent to mailto:email%40site.com and mailto:?to=email1%40site.com&to=email2%40site.com is equivalent to mailto:email1%40site.com%2C%20email2%40site.com
Also, the TO keywords don't have to come first. For example: mailto:?subject=bla&to=email1%40site.com&body=bla&to=email2%40site.com is also valid.
Right-click on the link, "Copy Email Address" and paste into the input to see if it matches the expected results.
Note: email addresses directly following "mailto:" are referenced as email addresses from the MAILTO keyword.