2.11 Package History

Version 1 of the email package was bundled with Python releases up to Python 2.2.1. Version 2 was developed for the Python 2.3 release, and backported to Python 2.2.2. It was also available as a separate distutils-based package, and is compatible back to Python 2.1.

email version 3.0 was released with Python 2.4 and as a separate distutils-based package. It is compatible back to Python 2.3.

Here are the differences between email version 3 and version 2:

The FeedParser class was introduced, and the Parser class was implemented in terms of the FeedParser. All parsing there for is non-strict, and parsing will make a best effort never to raise an exception. Problems found while parsing messages are stored in the message's defect attribute.
All aspects of the API which raised DeprecationWarnings in version 2 have been removed. These include the _encoder argument to the MIMEText constructor, the Message.add_payload() method, the Utils.dump_address_pair() function, and the functions Utils.decode() and Utils.encode().
New DeprecationWarnings have been added to: Generator.__call__(), Message.get_type(), Message.get_main_type(), Message.get_subtype(), and the strict argument to the Parser class. These are expected to be removed in email 3.1.
Support for Pythons earlier than 2.3 has been removed.

Here are the differences between email version 2 and version 1:

The email.Header and email.Charset modules have been added.
The pickle format for Message instances has changed. Since this was never (and still isn't) formally defined, this isn't considered a backward incompatibility. However if your application pickles and unpickles Message instances, be aware that in email version 2, Message instances now have private variables _charset and _default_type.
Several methods in the Message class have been deprecated, or their signatures changed. Also, many new methods have been added. See the documentation for the Message class for details. The changes should be completely backward compatible.
The object structure has changed in the face of message/rfc822 content types. In email version 1, such a type would be represented by a scalar payload, i.e. the container message's is_multipart() returned false, get_payload() was not a list object, but a single Message instance.
This structure was inconsistent with the rest of the package, so the object representation for message/rfc822 content types was changed. In email version 2, the container does return True from is_multipart(), and get_payload() returns a list containing a single Message item.
Note that this is one place that backward compatibility could not be completely maintained. However, if you're already testing the return type of get_payload(), you should be fine. You just need to make sure your code doesn't do a set_payload() with a Message instance on a container with a content type of message/rfc822.
The Parser constructor's strict argument was added, and its parse() and parsestr() methods grew a headersonly argument. The strict flag was also added to functions email.message_from_file() and email.message_from_string().
Generator.__call__() is deprecated; use Generator.flatten() instead. The Generator class has also grown the clone() method.
The DecodedGenerator class in the email.Generator module was added.
The intermediate base classes MIMENonMultipart and MIMEMultipart have been added, and interposed in the class hierarchy for most of the other MIME-related derived classes.
The _encoder argument to the MIMEText constructor has been deprecated. Encoding now happens implicitly based on the _charset argument.
The following functions in the email.Utils module have been deprecated: dump_address_pairs(), decode(), and encode(). The following functions have been added to the module: make_msgid(), decode_rfc2231(), encode_rfc2231(), and decode_params().
The non-public function email.Iterators._structure() was added.