Permanent flags not settable

[beepmeep]

Describe the bug
I seem to be unable to set Permanent Flags on a ftgate-IMAP Server.

When a '\delete' is issued over 'mailbox.Addflags' the client sends an empty Flag-Store Command to the Server when it should send 'UID STORE +FLAGS ( \deleted )' .

Platform (please complete the following information):

• OS: Windows
• .NET Runtime:
• .NET Framework: .net 6.0
• MailKit Version: 2.15.0

To Reproduce

            string host = "";
            string password = "";
            string account = "";

            var client = new ImapClient(new ProtocolLogger(Console.OpenStandardOutput()));
            client.Connect(host, 143, SecureSocketOptions.StartTlsWhenAvailable);
            client.Authenticate(account, password);
            Console.WriteLine("Capabilities = {0}", client.Capabilities);

            var inbox = client.Inbox;
            inbox.Open(FolderAccess.ReadWrite);
            Console.WriteLine("PermanentFlags = {0}", inbox.PermanentFlags);
            Console.WriteLine("AcceptedFlags = {0}", inbox.AcceptedFlags);

            var mm = client.Inbox.Search(SearchQuery.All);
            inbox.AddFlags(mm, MessageFlags.Deleted, true);
            inbox.Expunge();

            client.Disconnect(true);

Expected behavior
The mails should receive the '\deleted' flag and be expunged.

Protocol Logs

Connected to xxxxxxxxxxxxxxxxxxxxxxxxxxxx/?starttls=when-available
S: * OK IMAP4 IMAP4rev1 Server
C: A00000000 CAPABILITY
S: * CAPABILITY IMAP4 IMAP4REV1 AUTH=CRAM-MD5 AUTH=LOGIN IDLE XTRASH XLIST
S: A00000000 OK CAPABILITY completed
C: A00000001 AUTHENTICATE CRAM-MD5
....
S: A00000001 OK AUTHorisation successful
C: A00000002 CAPABILITY
S: * CAPABILITY IMAP4 IMAP4REV1 AUTH=CRAM-MD5 AUTH=LOGIN IDLE XTRASH XLIST
S: A00000002 OK CAPABILITY completed
C: A00000003 LIST "" ""
S: * LIST (\NoSelect) "/" ""
S: A00000003 OK List Completed
C: A00000004 LIST "" "INBOX"
S: * LIST (\NoInferiors) NIL INBOX
S: A00000004 OK List Completed
C: A00000005 XLIST "" "*"
S: * LIST (\Drafts) "/" "Drafts"
S: * LIST () "/" "INBOX"
S: * LIST (\Sent) "/" "Sent Items"
S: A00000005 OK List Completed
Capabilities = IMAP4, IMAP4rev1, Status, Idle, XList
C: A00000006 SELECT INBOX
S: * FLAGS (\answered \flagged \deleted \seen \draft)
S: * 3 EXISTS
S: * 3 RECENT
S: * OK [UNSEEN 6] Message 6 is the first unseen
S: * OK [UIDNEXT 9] Predicted next uid
S: * OK [UIDVALIDITY 1634801892] UIDs VALID
S: * OK [PERMANENTFLAGS (\seen \answered \flagged \deleted \draft \new)]
S: A00000006 OK [READ-WRITE] SELECT COMPLETED
PermanentFlags = None
AcceptedFlags = None
C: A00000007 UID SEARCH ALL
S: * SEARCH 6 7 8
S: A00000007 OK Search Complete
C: A00000008 UID STORE 6:8 +FLAGS.SILENT ()
S: A00000008 OK UID STORE completed
C: A00000009 EXPUNGE
S: A00000009 OK EXPUNGE completed
C: A00000010 LOGOUT
S: * BYE Server closing connection
S: A00000010 OK LOGOUT completed

Additional context

• I am able to set random custom-flags. I tried to 'cheat' the system by issuing the '\deleted' flag over the custom flag system, but MailKit seems to escape known permanent flags in custom-flags.
• I can delete E-mails fine if i use a telnet-connection.

[jstedfast]

The problem is that your IMAP server is using all-lowercase flag names and MailKit was not handling that correctly (It handles it correctly for everything other than flag names, funnily enough).

[jstedfast]

MailKit v3.0.0 has been released with this fix.