Your customer may complain sometime they cannot use their SMTP server to sending mails out. Most probably the ISP started blocking 3rd party smtp servers.
In this case you can provide an alternative port for your customers to send mails out.
Some customers having problem accessing your PLESK control panel using from their work but it working fine from their home. This is a case network admin at work is blocking the port 8443.
Here is a simple solution.
You can use your firwall to route all traffic coming to one port to another, no need to change or configure anything in the PLESK. no need to configure things again and again after every PLESK upgrade.
Just preroute the ports.
e.g: routing smtp port 25 to 3525
iptables -t nat -I PREROUTING -p tcp --dport 3625 -j REDIRECT --to-port 25
e.g: routing PSA access port 8443 to 3636
iptables -t nat -I PREROUTING -p tcp --dport 3636 -j REDIRECT --to-port 8443
if you are using APF firewall then just add following line to the preroute.rules file.
$IPT -t nat -I PREROUTING -p tcp --dport 3625 -j REDIRECT --to-port 25
$IPT -t nat -I PREROUTING -p tcp --dport 3636 -j REDIRECT --to-port 8443
Use it, test it. And I should not be held responsible for any misconfiguration of your firewall settings.