In what context are you working on those items? It might be, for example, that you are writing in a wordprocessor and you include in your text something that looks like an email address (e.g. a string of characters with '@' in the middle). A lot of software these days is, in my personal view, over-clever, and the computer might think you are writing an email when you aren't. The message is telling you that the computer doesn't know what program to use to send an email (so it's not so clever after all ;-) ) and I can't imagine why it would warn you about that unless it thinks that that's what you want to do.