For gmail at least it will be the same reason that it can't send to smtp.verizon.net - the outgoing connection needs to be encrypted - ssl
Don't know about yahoo as I've never attempted to send mail via that using a remote client.
For gmail at least it will be the same reason that it can't send to smtp.verizon.net - the outgoing connection needs to be encrypted - ssl
Don't know about yahoo as I've never attempted to send mail via that using a remote client.