Ok, The post I linked to does give the information you need but I will give a brief rundown of what you need to do.
You need to know:
Your network is set up only as a workgroup, not a Domain.
Name given to your Synology Diskstation
IP address of your Synology Diskstation. (Must be a fixed IP address on your LAN)
email domain you wish to use
Normally requests for email server IP's are done using DNS records. As you are only running a local mail server you need to tell each computer where to find the mail server as they cannot use DNS records. (Unless you are running a local DNS server of course)
The only difference between Roundcube and Outlook is that Roundcube knows were the mail server is located because it too runs on the same server. Outloook Express needs to know where to look to find the mail server, but you cannot set a numerical IP for a POP/IMAP server in Outlook Express, it has to be in domain name format. The way we achieve this is to edit the hosts file on each computer on the LAN. The hosts file is like a local DNS lookup, it will search there before it searches DNS servers.
Firstly, you need to know what email domain you are going to use. If your email address is "email@example.com" then you need to add "example.com" as the domain name in Mailstation. Also on each computer edit the hosts file. I'm guessing you're using windows so edit (or create if one does not exist) the text file located at "%systemroot% \system32\drivers\etc\hosts" Add the hostname of the DS and the local IP address in the format:
"local IP of DS" "Hostname" # Enter comment here so you know what it is. EG; Diskstation IP address and Hostname
"local IP of DS" mail.example.com # Enter comment here so you know what it is. EG: Mail Server
(leave out the quotation marks. The hosts file location given here is for Windows 2000, XP, Vista and Windows 7)
Now in Outlook Express you need to set the server names. For IMAP/POP3 and SMTP the server name is exactly what you entered in the hosts file on each computer. In the above example it would be "mail.example.com" Adjust as needed.
You will need authentication to the mail server so adjust this to your needs. Outlook Express should now be able to communicate with your Diskstation.
Repeat process on every computer on your LAN.
Hope this helps.