Export to E-Mail

I want to clarify a previous question about e-mailing from within Lr.

When I export a photo to email, and the "Send To: and Subject:

Window is opened the "Include caption metadata as a description label" is grayed out, even though I assigned a caption to the photo.

Is there a way to activate this check box?

I am using Outlook 2010 as my e-mail client.

Is this function not available in Outlook?

I do not use an internet service such as G-mail.

I'm curious, is this check box available in G-mail etc.

Thanks,

Roger