Use SocksiPy:
import smtplib
import socks
#'proxy_port' should be an integer
#'PROXY_TYPE_SOCKS4' can be replaced to HTTP or PROXY_TYPE_SOCKS5
socks.setdefaultproxy(socks.PROXY_TYPE_SOCKS4, proxy_host, proxy_port)
socks.wrapmodule(smtplib)
smtp = smtplib.SMTP()
...