Server does not find a message with a subject divided in two lines.

[NilsHoyer]

I have a mail on my IMAP server, I've downloaded with Thunderbird into a file. I start with the following lines:

From:
Date: Sun, 25 Aug 2024 12:13:18 +0200
Subject: [Test] Advertise
 (WindowsC02HFGPPDV13S-1-5-21-106835334-3766545784-1673647939-1000)
Message-Id: <6C1FGVPFXNU4.OT5FN99GEAZ5@macbookpro>
MIME-Version: 1.0

Now, I try to find it with following code:

var searchQuery = SearchQuery.SubjectContains("Advertise (");
var uniqueIds = _mailFolder.SearchAsync(searchQuery, Context.CancellationToken).Result;

The message will not be found. But Thunderbird displays the subject as well.

I wonder about the Subject is divided into two lines, but I'm not a Mail professional.

Inside the log, I found this line:

C: A00000008 UID SEARCH SUBJECT "Advertise ("

Am I right, you believe, this is a server bug?

Can I specify another search string, which finds this message?

I've sent this message with MailKit.

[jstedfast]

Yea, this is a server bug.

The Subject header that is split across 2 lines is perfectly legal, so there's nothing wrong with the message itself, but the server is clearly not unfolding the Subject value before doing a string match against it.

You might be able to work around this by doing:

var searchQuery = SearchQuery.SubjectContains("Advertise\r\n (");

But I wouldn't hold out hope.

[NilsHoyer]

Maybe your suggestion works, but I believe a better solution is to use less matching definition for the SearchQuery and use a regular expression after fetching the mail.

If I'm right, IMAP does not offer regular expressions or joker characters for the SEARCH command?

[jstedfast]

Unfortunately, IMAP does not support regular expressions or wildcard matching in the SEARCH command.

As you said, the best way might be to do client side matching.