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.