Giving a last review before committing. Thanks Michael for your eforts and your patience.

Don't you think this block would be more readable in this form?:
```python
# We need an email.Message object
if isinstance(content, EmailMessage):
    # convert content into an email.Message
    content = content.message()
elif not isinstance(content, Message):
    # For compatibility with existing code
    content = message_from_string(content)
```