Script reproducing this behavior:

```python3
from email.mime.text import MIMEText
msg = MIMEText('<div/>', 'html')
msg["To"] = "this.email.shouldnt.be.printed@nokia.com"
msg["To"] = "valid.email@nokia.com"
print(msg["To"])
```

stdout:
```
this.email.shouldnt.be.printed@nokia.com
```

I think this is the expected behavior, see https://docs.python.org/3.4/library/email.message.html#email.message.Message.__getitem__ for details:

> Note that if the named field appears more than once in the message’s headers, exactly which of those field values will be returned is undefined. Use the get_all() method to get the values of all the extant named headers.

This has been already discussed elsewhere, it may not be the best API but for compatibility reasons, we cannot change it anyway.

Here's how you can rewrite your code so it is more explicit:

Python 3.7.2 (default, Feb 12 2019, 08:15:36)
[Clang 10.0.0 (clang-1000.11.45.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from email.mime.text import MIMEText
>>> msg = MIMEText('<div/>', 'html')
>>> msg.add_header('To', "this.email.shouldnt.be.printed@nokia.com")
>>> msg.add_header('To', "valid.email@nokia.com")
>>> print(msg.get_all('To'))
['this.email.shouldnt.be.printed@nokia.com', 'valid.email@nokia.com']

Ok, so this is not a bug, I must missed this information in documentation.

In my defense this usage of `__getitem__` and `__setitem__` feels a little bit counter-intuitive.

Sorry bothering.