Relaying to a non-dsn capable server

When a message with NOTIFY=SUCCESS is relayed and the corresponding relay doesn't support DSN we generate a DSN with message "relayed" instead of "success" as per RFC3461.

Example: if a.org is a dsn capable server and b.org is NOT a dsn capable server then

echo "hello world" | mail -s dsn foo@a.org bar@b.org -Nsuccess

should generate two DSN where a.org sends it's dsn for foo@a.org and the local mta should generate a dsn for bar@b.org.