URLs in my emails not "clickable"

I think the way to get a web address clickable in a signature is to use html formatting,

AND

WLM 2011 seems to add a space automatically after the signature website address. I found that by deleting this space and then placing a manual carriage return (press enter) after placing the signature in the message, the signature website address is clickable. I have been unable to do this in the actual signature window itself.

I have found that even in messages formatted in plain text, adding that extra carriage return (pressing enter) makes the signature website address clickable--in most cases.

I experimented with this by sending test emails to comcast, yahoo and aol. Only the yahoo email's website signature was not clickable in plain text.

I have no idea why.