CC Proxy can work as mail proxy server for Outlook Express.

CCProxy is an easy-to-use proxy server software. Full supports Windows 98/ME/NT/2000/XP. For more details of CCProxy, please click here. Also you can download it here for a trial. CC Proxy Server allows all computers on the LAN access the Internet through a single Internet connection. You only need to install CC Proxy on the server which can access the Internet directly and other PC clients can connect to the Internet through CC Proxy. It will provide you a cost-effective Internet connection sharing solution without losing ease of use and peace of mind.

Suppose email address is .
Mailbox account: support
Password:***********
SMTP address: smtp.youngzsoft.com
POP3 address: pop3.youngzsoft.com
Proxy server address: 192.168.0.1

Launch Outlook Express.
Menu"Tools"--> "Accounts"--> "Mail" Tab

Click button" Properties". The general info should be shown as below:

Go to "Server" tab. The info should be shown as below.
Pay attention to the account name of "Incoming Mail Server".

Click button "Settings".
If your original mail server requires authentication when sending emails,
you need to configure as below.
Pay attention to the account name. It is "support#smtp.youngzsoft.com" here.

If your original mail server doesn't require authentication when sending emails,
then configure as below.
Pay attention to the account name. It is "#smtp.youngzsoft.com" here.