Thank you for you reply,

1. Is the option "This server is a reverse proxy lookup target" will effect the DNS check "reject_unknown_client", because I forced zimbra to check the reverse IP address when it receive any email in order to improve and stop the spam..

2. As per my searching, I noticed that more than 90% of the topics suggested to uncheck the "Enable clear text login" and use the SSL for sending and receiving with TLS authentication only. (What is your suggestion)

What is need exactly is to send and receive via POP3 and SMTP with TLS Authentication and the same time I need to adjust the security from the server side to avoid the spam and lab lab lab lab .....