email_fields = {'txt': ('subject', 'plaintext_body'),
                'html': ('html_body',)}

>>> get_my_list(email_fields)
[('txt', 'subject'), ('txt', 'plaintext_body'), ('html', 'html_body')]

foo = starmap(product, email_fields.items())
chain.from_iterable(foo)

[('t', 'subject'),
 ('t', 'plaintext_body'),
 ('x', 'subject'),
 ('x', 'plaintext_body'),
 ('t', 'subject'),
 ('t', 'plaintext_body')],
 ('h', 'html_body'),
 ('t', 'html_body'),
 ('m', 'html_body'),
 ('l', 'html_body')]

foo = [product([ext], field) for ext, field in email_fields.items()]
chain.from_iterable(foo)

[(k, v) for k in email_fields for v in email_fields[k]]

[('txt', 'subject'), ('txt', 'plaintext_body'), ('html', 'html_body')]

print [(k, item) for k in email_fields for item in email_fields[k]]
# [('txt', 'subject'), ('txt', 'plaintext_body'), ('html', 'html_body')]