The s/mime signature that alpine generates fails to include any intermediate chained CA certs when the user's personal cert
was created from an intermediate CA. This causes signature verification failures at the recipients, unless the recipients happen
to have the necessary intermediate chained CAs in their local CA store. ("Couldn't verify S/MIME signature: certificate verify error").
In the file "pith/smime.c" the call to "PKCS7_sign" has the third argument set to "NULL". This should be a "STACK_OF(X509)" structure
populated with intermediate chained CA certs for the case that the user's signing cert has them.