imap.Connect(txtmailIncMailSrver.Text.Trim()); imap.StartTLS();
after excecuting the imap.StartTLS(), it throws an error like unknown command
Please provide me solution
It looks like your server is not supporting STARTTLS command (explicit SSL/TLS mode, where you first connect on plain text port and then secure the connection).
Consider using ConnectSSL instead:
imap.ConnectSSL("imap.example.com"); imap.UseBestLogin("user", "pass");
You can find more details on how to use SSL/TLS and IMAP here: https://www.limilabs.com/blog/use-ssl-with-imap