Why can I not use a web-based mail program of my choice?
why not just create a mailto-link that can be copied into any mail-software?
it's a pity I cannot use the service ...

I will add an additional mailto link on the next version...
Until then, you can click the button (which is anyhow implemented as a mailto link), and copy the to/subject/body from the message that opens in your local mail client, and paste it into your webmail.
Take care: Mistakes in the copy/paste may damage the message.