Fetchmail only retrieve email from the ISP and it can place it either in the IMAP store or route it to Procmail. It is configured bu going to Yast->Network Services->MTA. Fetchmail delivers to the local MTA. So fetchmail itself isn't the MTA, it only acts as the transporter from an external mailbox to your local MTA, in this case Postfix.
Your correct, fetchmail is sort of a pretend MTA, a 'shim' if you will to get mail from point A to point B (point B bieng an MTA).
Postfix seems to both receive and send email but its exact purpose is unknown, other than it is a MTA. You said it, it is an MTA - a thing that RECEIVES and SENDS mail - but does not STORE mail or provide ACCESS to mail. .....So, like Fetchmail is the transporter of incoming mail does Postfix, how is outgoing email sent? Does Postfix use an inbuilt SMTP to transport sent mail?
Postfix is an MTA, thus SMTP is its native tongue.
clamav is an antivirus mechanism which will be used as Windows clients(my laptop) will be accessing the IMAP store. Yep. What would invoke Clamav, Postfix as it is the processor in-between the external source and internal IMAP store? Postfix should encapsulate both SA and CLAM, you shouldn't have to worry about them. So Postfix will invoke both SA and Clamav and then pass the mail to the IMAP store.
Yes, postfix recieves mail and runs it though a 'milter' process, which may contain SA, clamav, whatever.... gets the mail back and then drops it somewhere.
Sendmail isn't nearly that scarry, except for ninnies that can't spell M-4. No one edits sendmail.cf. Why would I need sendmail, surely I have all the above packages to receive(Fetchmail) and send(Postfix) mail?
You don't need sendmail (most likely). I'm just saying it isn't any more or less scary than basically any other MTA. MTAs are very complicated chunks of software, complexity is the norm.