Bear in mind that nearly all of the things there can be done via YaST. In general I would encourage you to use YaST to do all the configuration and not webmin. Postfix under openSUSE is interesting in that if you modify main.cf yourself, YaST control ceases to be effective. In practice I have been able to configure postfix through YaST, or /etc/sysconfig/postfix (same thing, but using text editor).
There are other aspects not covered by that howto, come back with questions when you get to that point.
I suspect those are for fetchmail to use. There are a couple of ways you can go about getting incoming mail.
Get fetchmail to login to a remote mail server (e.g. your ISP, or gmail) and download the mail and inject them into the local postfix server. This method has the simplicity of doing what you would normally do with a mail client like Thunderbird. You also don’t have to worry about keeping the server up, filtering spam, attacks by bots and that sort of thing, your mail service provider does those.
Set up your mail server to listen on your external address and accept incoming mail from the Net. This method requires that you set up a MX record for your domain that points to your IP address. If you don’t have a static IP address, then you need a dynamic IP registrar. You also have to keep your machine up all the time or lose mail if you are too long off the Net. You will also have malware on 0wned machines probing your server, and filling the logs, which is a nuisance. If your mail address became public, then you will also get spam sent to you and you will have to do some filtering. The advantage is that you get full control over your mail domain, you can set up as many accounts as you like, and you don’t have to accept your ISP’s domain name as your mail domain.
If you don’t understand what is entailed in 2, then I suggest you stick with 1 until you get more experience.
I do not know what ken_yap means, but he was right to be a bit ambiguous here. This depends entirely on the sender and there are many different mail transport agents out there. A simple spam bot may make one try and then give up, while a decent MTA tries for 5 days to deliver the mail message.
In principle a mailserver must always be up and running. You can never switch it off. In practice you will have some downtimes. To compensate for this you set up a second mailserver for your domain with a separate MX record and a lower priority. When your primary is not reachable senders will deliver to the backup server (yes, they are running both all the time) and the backup server will store the message for up to 5 days and forward it to the primary once it is reachable again.
Let me ramble a bit further. Many assume that this is all done with postfix because this is the default nowadays for openSUSE. I still use sendmail which is well supported and can be configured with yast equally well. It is a matter of taste and of what you know best – an important factor, because handling mail is more a bit of an art than a technique. My suggestion is that you read a lot and come back with questions whenever you got stuck.
-) Samba as PDC for connecting windows laptop (2) and Box (1 + 1 linux server). I made a try two years ago with success.
-) backup for everyone.
-) mail for everyone ( and fetching from multi ISP )
*) For speed access
*) An individual organization which we agree.
*) For fun but not in stupid way
And perhaps a web server if necessary in future ( not this year )
So if I understand I must chose an other way ( not installing a mail server ) if I plan to shutdown my server every night.
I was expecting to configure my mail server pointing to my ISP and acting as something like an incoming relay for incoming mail and acting as an outgoing relay for outgoing mail.
I have no fixed IP adresse, but I know that I can use DynDns.