Use Imap.PeekMessageByUID instead of Imap.GetMessageByUID when you download the emails.
using(Imap imap = new Imap())
{
imap.ConnectSSL("imap.server.com");
imap.UseBestLogin("user", "password");
imap.SelectInbox();
List<long> uidList = imap.Search(Flag.Unseen);
foreach (long uid in uidList)
{
IMail email = new MailBuilder()
.CreateFromEml(imap.PeekMessageByUID(uid)); //<-- Peek
string subject = email.Subject;
}
imap.Close();
}