How to limit incoming connectivity to a particular port (email)

I have an upstream email spam filtering provider that is listed as our mx record, then they pass our email on to us at our gateway/firewall router (ClearOS Business Edition 7.4-1) on port 25.
email from their servers is the ONLY inbound email we want to receive...

Other misc info to paint the whole picture.
our email server is another server on our lan. I want to configure the clearOS box, that CURRENTLY forwards port 25 traffic to our email server via port forwarding.

What is the best way to accomplish this on our clearOS box, which is our gateway firewall? I am assuming this is a custom firewall entry, but maybe there is an app already for this.
Any and all help is appreciated.
Sincerely,
George Miller

Note: clarification, I am now off of all blacklists. when I send an email to myself from gmail, I get nothing.. no return receipt, nothing, but I never get the email in my inbox of the webmail.

I did a postsuper -d ALL and it took about 3 hours for it to clean out all the (1040981 total) messages.
after an hour or two it filled back up to about 500 messages
I turned off everything accept http and https on the firewall and now it does not seem to be filling back up.

this is an educational thing for me... not a production email server, rather one I had set up for back office so i have some time.

would really appreciate some guidance on how to troubleshoot and figure out how they are getting in to do this.

I looked in the logs, but am not sure what to look for.

I am using the paid version of roundcubemail. only two email users on the system....

i reappeared on the black list, but disappeared again after closing up the firewall as described above.

Need Help with Email Server

My server was recently on a black list. I found that there were a bunch of PHP files uploaded to the web site in various directories. I cleaned out all the bogus files and reset the permissions on the web server and I have stopped sending out spam, and the web server appears to be clean. I changed the web sql and server root passwords and updated the joomla config files and the web sites are working fine.

HOWEVER, my email server (prevshop.com) dos not appear to be working any more. It did work fine. Someitmes when I open the web admin panel in the browser and look at the SMTP menu, in the upper right corner I see beside "additional info" "connection failed" in red letters. when i run TOP from the command line the SQL process is pretty high (99-100%) still.

I also have roundcube web mail which use to work fine, but now when I run it it often tells me there are too many sql connections, and when the email list comes up, the date column is blank among several emails in my inbox.

My gut feeling is that something is wrong with the system mysql but I have no idea how to troubleshoot it.

Here is an output of my TOP

[root@prevshop ~]# top
top - 15:19:54 up 3 days, 22:05, 3 users, load average: 11.86, 12.15, 12.42
Tasks: 663 total, 1 running, 661 sleeping, 0 stopped, 1 zombie
Cpu(s): 12.3%us, 13.8%sy, 0.1%ni, 38.4%id, 35.5%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 7539108k total, 6643372k used, 895736k free, 618688k buffers
Swap: 8384444k total, 286880k used, 8097564k free, 1351436k cached

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
20376 system-m 20 0 2766m 140m 4808 S 100.1 1.9 914:42.95 system-mysqld
20459 dansguar 30 10 123m 4044 440 S 2.0 0.1 3:04.72 dansguardian-av
1089 root 20 0 0 0 0 D 0.3 0.0 1:57.19 flush-9:2
1862 ldap 20 0 1975m 109m 2800 S 0.3 1.5 36:15.52 slapd
5300 root 20 0 156m 2796 2268 S 0.3 0.0 1:00.33 X
27819 amavis 20 0 317m 55m 3284 S 0.3 0.8 0:00.24 amavisd
27826 root 20 0 13536 1684 904 R 0.3 0.0 0:00.05 top
27835 amavis 20 0 317m 55m 3340 D 0.3 0.8 0:00.14 amavisd
30297 postfix 20 0 83652 4748 2996 S 0.3 0.1 0:07.06 trivial-rewrite
1 root 20 0 21448 1108 896 S 0.0 0.0 0:02.80 init
2 root 20 0 0 0 0 S 0.0 0.0 0:00.00 kthreadd
3 root RT 0 0 0 0 S 0.0 0.0 0:03.69 migration/0
4 root 20 0 0 0 0 S 0.0 0.0 0:20.34 ksoftirqd/0
5 root RT 0 0 0 0 S 0.0 0.0 0:00.00 stopper/0
6 root RT 0 0 0 0 S 0.0 0.0 0:00.33 watchdog/0
7 root RT 0 0 0 0 S 0.0 0.0 0:04.88 migration/1
8 root RT 0 0 0 0 S 0.0 0.0 0:00.00 stopper/1

Any help is appreciated,
Thanks,
George