Mailing list subscribe and unsubscribe links

This is related to the fix done in https://issues.apache.org/jira/browse/MPIR-398

We were using the mailing list a little different that what was expected.

So for us we don't have a mailing list, but we do have yammer and we were using the subscribe and unsubscribe links to subscribe and unsubscribe to our yammer group. We use the yammer group as a mailing list.

When we took 3.1.2 it broke our links because now they have a mailto: before the link.

Curious if it would be possible to enhance this code to detect if the value in the unsubscribe and subscribe is an email address or a URL. If it is an email address then add the mailto: other wise don't.