Invalid local-part at offset 0

Hello,

I am getting just another error with “Invalid local-part at offset 0” Using the code:

public class EnvelopeShould
{
    [Fact]
    public void Parse()
    {
        var rawEnvelope =
            "(\"Tue, 23 Dec 2014 17:03:26 +0100\" \" Merry Christmas!\" ((\"Firstname LastName\" NIL \"f.lastname\" \"ex.com\")) ((\"Firstname LastName\" NIL \"f.lastname\" \"ex.com\")) ((\"Firstname LastName\" NIL \"f.lastname\" \"ex.com\")) ((NIL NIL \"\" \"localhost\")) NIL NIL NIL \"<D1329FAF41FC457FA57F0427E6AEB5E0@example.com>\")";

        var parsed = Envelope.TryParse(rawEnvelope, out var envelope);
        Assert.True(parsed);
    }
}

Seems like this part is making issue: ((NIL NIL "" "localhost"))

if I change it to ((NIL NIL "localhost" "localhost"))

it works fine… however have no idea about details here. Envelope is received from IMAP server. Any advice?