We don't have a list of IMAP servers which supports keywords feature, sorry.
However, you can simply try it for any IMAP server by the
If you have at least one mail in the INBOX folder, you can do it simply like this:
var client = new Imap();
client.SetMessageFlags(1, ImapFlagAction.Add, ImapMessageFlags.Keywords, "Test-KW");
ImapMessageCollection list = client.Search(ImapSearchParameter.Keyword("Test-KW"));
If the IMAP server supports keywords feature, the
list collection would not be empty.