Mikrotik SMTP Traffic block except mail server

RouterOS Forwarding Protocols
1

Hi guys im in desperate need of help

i have a Mikrotik Haplite in a clients business which is connected to a switch, the mikrotik runs a PPPoE and has a ip of 10.6.1.98 and a lan of 172.16.5.199
my clients are running a Exchange mail server with ofcourse port 25, they want me to block ALL outbound SMTP traffic on your the mikrotik router from all internal IP’s 172.16.5.0/24 except 172.16.5.5, they want all the internal pcs to send the emails to the exchange server which will then send it out, they also stil want to be able to telnet into 10.6.1.98 via port 25 from the outside of the network which will be done via our public ip, but i cant manage to block all outbound smtp traffic internally only allowing 172.16.5.5

i really need some help guys please, here is a diagram of the current setup

2 
chain=forward protocol=tcp src-address=172.16.5.5 dst-port=25 action=accept
chain=forward protocol=tcp src-address=172.16.5.0/24 dst-port=25 action=drop
3

i tried those lines, but it doesnt work

i tried chain=forward src-address=!172.16.5.5 protocol=tcp dst-port=25 action=drop

aswell

4

It should work. Post Your firewall rules here.