Punycode encoding being improperly used for email local parts

[st-abarker]

Describe the bug When working with a message which has a MIME header that contains an email address with non-ASCII characters in the local part, operations that require a FormatOptions argument incorrectly process the email address if FormatOptions.International is false. Specifically, the local part is being encoded with punycode.

Platform (please complete the following information):

• OS: Windows
• .NET Framework: .NET 8
• MimeKit Version: 4.1.0

To Reproduce Steps to reproduce the behavior:

1. Build a MIME message.
2. For one of the email headers (e.g., From, To, CC, etc.) include an email that has non-ASCII characters in the local part. For example, उदाहरण@example.com
3. Write the message to a file using the default FormatOptions.
4. Open the message in a text viewer.
5. Note that the email provided in step 2 has the local part encoded using punycode. For the provided example, that would be xn--p1b6ci4b4b3a@example.com

Expected behavior In this scenario, I would expect an exception to be thrown indicating that the message cannot be represented without plain ASCII headers.

Code Snippets

using var message = new MimeMessage();
message.FromFrom.Add(new MailboxAddress("Example", "उद\u093eहरण@example.com"));
// Finish building the MimeMessage

using var fs = new FileStream(filePath)
message.WriteTo(FormatOptions.Default, fs);

Additional context Based on RFC 5890, RFC 6530, and other related RFCs, only the domain part of an email address can be converted between Unicode and ASCII-only formats. These RFCs do not define a method for converting between a Unicode local part and an ASCII-only local part, leaving such decisions up to implementors. RFC 6530 section 4.3 indicates that a a user with an internationalized email address may have all-ASCII address(es) as well, either through having multiple accounts or aliases, but there is no way for an external system to know of the relationship between those accounts.

[jstedfast]

Yea, I recently discovered this when looking at https://github.com/arnt/eai-test-messages a week or 2 ago.

I just hadn't gotten around to fixing it.