In Depth Guide

This document describes the API in a stage-by-stage basis. It is useful as a book-like, gentle technical introduction to the higher and lower level APIs. Before reading this guide it is recommended that you browse through the Quickstart as it will give a very high level introduction to Maithon. If you want to look for some method or class, go to the API section.

Envelopes and MIMEs

The Envelope class actually wraps around multipart MIME objects. This means they can be composed of multiple other MIME objects, e.g. plaintext MIMEs, MIMEs containing HTML data, images, etc, which provides the perfect building block for a composable class. Envelopes are made of two parts- like a real life envelope, a “stamp”, or headers, and an enclosure, made of other mime objects:

from mailthon.envelope import Envelope
from mailthon.enclosure import PlainText

e = Envelope(
    headers={'X-Key': 'Value'},

An interesting thing to take note of is that envelopes can be enclosed within envelopes. Concretely speaking, Envelopes consist of the Headers class and a list of Enclosure objects:

>>> e.headers
{'X-Key': u'Value'}
>>> e.enclosure
[<mailthon.enclosure.PlainText object at 0x...>]

You might have noticed that the Value string that was set was changed to a Unicode value. Why is that so? This is because internally the Headers class decodes bytes values that we throw at it into Unicode objects, freeing the developer from any headaches about encoding. You can read more about these design decisions at the Design Decisions section.

Now that we’ve looked at the higher level API of the Envelope class, let’s plunge deeper into madness and look into how it generates MIME objects with the mime() method:

>>> e.mime()
<email.mime.multipart.MIMEMultipart object at 0x...>
  1. Generates a MIMEMultipart instance and attaches each of the enclosures with the attach() method. Conceptually this is what you’d do with a real envelope- put each of the content into the enclosure of the envelope.
  2. Puts a stamp on the envelope- sets the headers onto the envelope object. This is done via the prepare() method of the headers object, which handles setting the appropriate headers- e.g. it ignores the Bcc headers to save you from embarassment and also to make Mailthon compliant with RFC 2822.

Disecting Enclosures

Conceptually the Envelope and Enclosure classes are the same- they are both made out of headers and some content. API-wise, they are also nearly identical- they both provide the same mime() method. And you are right! Here we see that the enclosure objects do in fact have almost the same attributes:

>>> plaintext = PlainText('content')
>>> plaintext.headers
>>> plaintext.mime()
<email.mime.text.MIMEText object at 0x...>

However, speaking from a responsibility perspecitive, here is where they differ. Envelopes have the concept of senders and receivers- and must keep track of them. Enclosures however, are like a superset of envelopes- an envelope can be an enclosure, but not the other way round, (at least, without some tricks).

All Enclosures have a content attribute that represents the content of the enclosure. This is once again something that the envelope object doesn’t have:

>>> plaintext.content

The role as a MIME-preparing class is the same. As mentioned earlier, both classes have the mime() method which prepares a MIME object- needless to say different subclasses of the Enclosure class handle different mimetypes, e.g. PlainText handles text/plain content. Similarly this is what an enclosure class does when it’s mime() method is called:

  1. Prepare the MIME object. For PlainText enclosures this returns a MIMEText object. For Binary enclosures the method returns a MIMEBase object which is a lower level but more configurable and flexible version of the MIMEText class.

  2. Apply the headers. Conceptually this is where the envelope analogy breaks down- you don’t usually have stamps inside enclosures, but let’s pretend that didn’t happen. The Enclosure object is designed in such a way such that the subclasses will not need to worry about applying the user’s headers. Essentially what the mime() method looks like is:

    def mime(self):
        mime = self.mime_object()
        return mime

    Which means that you usually do not have to worry about any headers that you’ve set not being applied to the generated MIME objects. So if you were to subclass the enclosure class:

    class Cat(Enclosure):
        def mime_object(self):
            return make_mime(self.cat_name)

    Which prevents you from shooting yourself in the foot. Or other parts of your body. Also it makes sure that, most of the time, you get the benefit of having the Mailthon infrastructure supporting your back- the main example being free of having to worry about encoding.

Few Sips of SMTP

How in the world, you ask, do you have tricks to make the Enclosure class to behave like an envelope? The Oracle answers, via the runtime modification of attributes which may cause headaches in production; but hey, let’s try them anyways:

enclosure = PlainText('something')
enclosure.mail_from = u''
enclosure.receivers = [u'', u'']

def string(self):
    return self.mime().as_string()

enclosure.string = string

Note that the mail_from and receivers attributes having Unicode values is absolutely necessary, and we’ll see why when we talk about then later when we explore the Postman object. For now, assume that they will be properly encoded by Mailthon. When we pass the enclosure we’ve mutated to a Postman instance, it’ll happily send it off:

>>> r = postman.send(enclosure)
>>> assert r.ok

Questioning our identity

Notice the mail_from attribute- it is not named something like sender. Why is that so? It is named such that it is synonymous with the SMTP MAIL FROM command. This is what is sent by a vanila (without any middleware) Postman instance in a typical SMTP session:

 <mime data>

Note the highlighted line- the address passed to the MAIL FROM command is the ‘true’ sender. For example you begin your letter with something along the lines of “From XXX”. The postman doesn’t care about whatever you wrote in there. He may, however write down your name somewhere for bookeeping reasons. The address passed to the MAIL FROM command is, essentially, your ‘true’ name. More info about this can be obtained by reading RFC 2821.

Usually you are doing the sane thing- you are sending from the same email address that you are claiming to send from (i.e. the one you set in the headers argument to the Envelope class). But if you wish to do so, you can change the ‘real’ address. There are two ways to do it:

from mailthon.headers import sender

envelope = Envelope(
    headers=[sender('Fake <>')],
envelope.mail_from = u''

However if you want the inferred sender (the one that was obtained from the headers) you can still do so via the sender attribute. You can read more about the behaviour of the mail_from attribute.

The headless MIME

In an ideal world, the SMTP protocol speaks Unicode and we can all throw poop emojis around at each other while pretending to get our work done. But that is sadly not the case. SMTP is a protocol which only understands bytes, and was invented way back in 1982 when nobody cared about characters outside the English alphabet.

As a result, the simple ASCII encoding stuck and was used as the de-facto standard for emails and most other protocols. However, SMTP, given that it does only operate in bytes, does allow you to simply do:

Subject: 哈咯 (Hello)

But some clients will not be able to read it if they are expected something encoded in ASCII, and suddenly get some UTF-8 value, and is likely to end up with Mojibake.

Instead, we must specify the encoding, and then rewrite all of the code points of the string so that it is ASCII-encoded. So your beautiful characters end up looking like:

>>> from email.header import Header
>>> Header(u'哈咯 (Hello)', 'utf-8').encode()

Not very nice, nor human readable. So rather than having you manually encode everything, Mailthon insists on having everything in Unicode. This makes everything a lot easier- extracting and encoding addresses, equality comparisions, etc. So the job of the Headers class (specifically, the UnicodeDict class) is to handle all this for you:

>>> from email.message import Message
>>> from mailthon.headers import Headers
>>> headers = Headers({
...    'Subject': u'∂y is not exact',
... })
>>> mime = Message()
>>> headers.prepare(mime)
>>> mime.as_string()
'Subject: =?utf-8?q?=E2=88=82y_is_not_exact?=\n\n'

For the record, it’s actually the Message class that does all the heavy lifting- for space saving and efficiency reasons, Mailthon simply supplies it with the Unicode string and it determines whether to encode with ASCII or UTF-8.

IDNA and Friends

Turns out that there is now a format for encoding domain names with non-ASCII characters in them, specified in RFC 3490 and usually referred to as IDN or IDNA. For a real life example: é.com. This gives us a pleasant surprise if we try to encode everything with UTF-8, the silver bullet to our Unicode encoding woes:

>>> u'é'.encode('utf8')
>>> u'é'.encode('idna')

A short detour on the format of email addresses- they are made up of two parts, separated by the first occurence of the ‘@’ symbol.

  1. Local-Part which can be UTF-8 encoded as per RFC 6531. The local part is not really important to the sending server who you are sending it to, rather it is more concerned with which server you are sending it to.
  2. Domain-Part which should be IDNA-encoded. Although servers which are compliant with both RFC 6531 and RFC 6532 can accept Unicode-encoded domain names, the pessimistic guess would be that most aren’t, so for the time being we are encoding in IDNA.

Putting it all together we have something like the following function:

def stringify_address(addr):
    localpart, domain = addr.split('@', 1)
    return b'@'.join([

But Mailthon already has a more robust implementation available in the form of the stringify_address() function, and is automatically used by the Postman class when sending envelopes. Via the sendmail() method. Essentially, the following:

def send(smtp, envelope):
        [stringify_address(k) for k in envelope.receivers],

Which explains why the addresses specified in the mail_from and receivers attributes must be Unicode values instead of byte strings since mixing them up will cause issues in Python 3.

The Postman Object

The Postman class is responsible for delivering the email via some transport, and is meant as a transport-agnostic layer to make sending emails via different protocols as painless as possible. Let’s start by creating a postman instance:

>>> from mailthon.postman import Postman
>>> postman = Postman(host='', port=587)

The Mutation Phase

The transport attribute. This is the actual “transport” used to send our emails over to a real server. To implement a transport it turns out that we need, at the very least, to have the ehlo, noop, quit, and sendmail methods:

class MyTransport(object):
    def __init__(self, host, port): = host
        self.port = port
        self.connection_started = False

    def check_conn(self):
        if not self.connection_started:
            raise IOError

    def noop(self):
        return 200

    def ehlo(self):
        self.connection_started = True

    def sendmail(self, sender, receipients, string):
        return {}

    def quit(self):
        self.connection_started = False

Next all we need to do is replace the tranport attribute with the class object that we’ve just created. Although this is not recommended as I recommend subclassing to change the transport being used we will do it anyways:

postman.transport = MyTransport

The response_cls attribute will contain a custom response class. We will create our own response class as well:

class Response(object):
    def __init__(self, rejected, status):
        self.rejected = rejected
        self.status_code = status

    def ok(self):
        return self.status_code == 200 and \
               not self.rejected

If you haven’t noticed, the __init__ method of our custom response class matches perfectly with the return values of the sendmail and noop methods from the MyTransport class, respectively. They are called by the Postman class like so:

def deliver(self, conn, envelope):
    rejected = conn.sendmail(...)
    return self.response_cls(rejected, conn.noop())

Now we just have to change the response class on the postman object we’ve created. Once again I recommend subclassing to change these attributes but for this experiment we’ll change them in runtime:

>>> postman.response_cls = Response

Putting it all together

Next we’ll send an envelope “across the wire” using our mutated postman object with our custom transport and response classes:

>>> r = postman.send(envelope)
>>> assert r.ok

But that doesn’t give us very much knowlegde of what happens underneath the hood. The send() method is simply a veneer over the lower level connection() and deliver() methods. Let’s recreate the send method:

>>> with postman.connection() as conn:
...    print(conn.connection_started)
...    r = postman.deliver(conn, envelope)
...    print(r)
<__main__.Response object at 0x...>

Basically what the connection() context manager does is that it manages the (usually SMTP) session for you. It is roughly implemented as:

def connection(self):
    conn = self.transport(, self.port)
        yield conn

Which closes the connection regardless of whether the sending operation is a success. This is important to prevent excessive memory and file-descriptor usage from the open sockets. You can verify that the connection as closed:

>>> conn.connection_started

Which is changed to False due the the context manager calling the quit method once the block of code within the with statement has finished executing. If you would like to find out how all of this is implemented you can take a look at the source code.

Middlewares and Middlemen

One of the more powerful features of Mailthon is the ability to add middleware- which are basically functions that allow for certain features, e.g. TLS, Auth which provide for TLS and authentication, respectively. Let’s make our own middleware to see how all of this is done:

def my_middleware(must_have=()):
    def func(conn):
        for item in must_have:
            assert hasattr(conn, item)
    return func

Then we need to put our middleware in what’s known as a middleware stack. It is basically a list of callables which will be invoked with the transport object. Using our Postman class:


Which will add the closure into the middleware stack and assert that the transport object has the quit attribute/method. More powerful middleware can certainly be programmed via classes, the recommended way if you want to make extensible middlewares is to subclass from the Middleware class:

from mailthon.middleware import Middleware

class MyMiddleware(Middle):
    def __call__(self, conn):

The registered middlewares will be called by the connection() method to set up the connection. If any exception is raised, the connection is automatically closed.