"numbchild@gmail.com" <numbchild@gmail.com> wrote:
> I'm unable to send email with smtpmail.el

What do you use then?  mstmp(1) or something alike?  What is the reason behind seeking a switch to smtpmail.el?

> I think it might be because of the internet censorship firewall in China.

This made me quite curious: why to choose a mail provider that is inaccessible without tricks?  There is a lot mail providers besides Google.

> So I decided to use a proxy for smtpmail. I have not found the smtpmail options for setting proxy.

IIRC, Emacs have a very rudimentary support of proxies.  Namely, there is no way to proxy a TLS stream, only a cleartext one.

In other words, there is really none.