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[grarpamp]

His keys load fine but none of his signed messages verify.

See if mailvelope can even read / write, cut / paste, signed
messages properly with test files on disk.

Try setting yahoo sending / compose options to disable html mail
and emoticons, enable stuff like "plain text" "ascii" "rfc <whatever>".
Try sending each setting to self for verify test.

Or switch from webmail to a local SMTP, IMAP / POP client
such as any of the ones posted earlier and configured to use
these settings...

https://help.yahoo.com/kb/SLN4075.html
https://help.yahoo.com/kb/SLN4724.html

Make backups, "keep messages on server", etc as prudent.