Build an Authenticating SMTP Client
Python 3 864 bytes

Extremely quick and dirty. Works with AWS SES and Yahoo!. Could not get it to work with Postmark (requires STARTTLS) or Gmail (requires further authentication outside of the SMTP connection).

import socket,ssl
from base64 import*
N=b'\x00'
U='utf-8'
E=lambda s:s.encode(U)
D=lambda b:b.decode(U)
B=lambda s:D(b64encode(E(s)))
A=lambda f,t,j,b:E('\r\n'.join(['DATA','Subject: =?UTF-8?B?{}?=','To: &lt;{}&gt;','From: &lt;{}&gt;','MIME-Version: 1.0','Content-Type: text/html; charset=utf-8','Content-Transfer-Encoding: base64',B(b),'','.','']).format(B(j),t,f,b))
X=lambda s,e:s.startswith(e)
def S(h,u,p,t,f,j,b):
    c,F,W=ssl.wrap_socket(socket.create_connection((h,465))),lambda m:E(m if'amazonaws'in h else'&lt;'+m+'&gt;'),lambda m:c.write(m+b'\r\n')
    def R(d,o=[]):
        x,y=c.read(),str(d)
        if not(X(x,E(y))or filter(lambda i:X(i,X(x,E(y))),o)):raise RuntimeError(y+' '+repr(x))
    R(220);W(b'EHLO x');R(250);W(b'AUTH PLAIN '+b64encode(N+E(u)+N+E(p)));R(235);W(b'MAIL FROM: '+F(f));R(250);W(b'RCPT TO: '+F(t));R(250);W(A(f,t,j,b));R(250,(354,));W(b'QUIT')


Invoke like S(hostname, username, password, to, from_, subject, body)

Expect bugs and changes (kind of got bored so it feels slightly half-baked).