Let's try again...Buy Bitcoin NOW.

[grarpamp]

Maybe test it independant of yahoo at this online test thing
from mailvelope...

https://demo.mailvelope.com/


Don't know what / if your requirement is to stay with
mailvelope vs any standalone mail client that speaks
standard SMTP, IMAP / POP to yahoo's servers,
plus some added bits for pgp.

If you missed them the first time, here's a list of tools
you could check out if mailvelope fails to serve...

https://www.mozilla.org/firefox/
https://www.enigmail.net/
https://www.mailpile.is/

https://www.gpg4win.org/
https://gnupg.org/

All these OS probably also supply firefox + enigmail,
and a bunch of other apps...

https://tail.boum.org/
https://www.whonix.org/
https://www.qubes-os.org/
https://subgraph.com/

There are also some new email providers that do
offer integrated pgp webmail, but they supply the code
to your browser, or in their system, both of which have
some potential to turn out bad, above and beyond the
third party tools listed above.