Potentially false 'Illegal characters in header field name' error

[sliekens]

Describe the bug
Hello,

I'm having problems with headers containing a period (.) like X-DCC-scarlet.be-Metrics. I'm hitting this exception:

I glanced at the ABNF grammar for header names and If I'm not mistaken, periods should be allowed?

The error should be thrown by the following code:

MimeKit/MimeKit/Header.cs

Lines 192 to 195 in 0913a82

	for (int i = 0; i < field.Length; i++) {
	if (field[i] >= 127 || !IsAsciiAtom ((byte) field[i]))
	throw new ArgumentException ("Illegal characters in header field name.", nameof (field));
	}

Platform (please complete the following information):

• OS: [e.g. Windows, Linux, MacOS, iOS, Android, Windows Phone, etc.] Debian 11
• .NET Runtime: [e.g. CoreCLR, Mono] CoreCLR
• .NET Framework: [e.g. .Net Core, .NET 4.5, UWP, etc.] .NET 6
• MimeKit Version: 4.1.0

Expected behavior
I think it should be possible to create an instance of Header even if it contains a period.

Code Snippets

var header = new MimeKit.Header("X-DCC-scarlet.be-Metrics", "whatever");

Additional context

For context: this is a header used by a Belgian internet service provider, https://www.scarlet.be. I guess they add headers like this for their own telemetry.

My use case is converting Outlook MSG files that contain this header to Mime messages.