I have a question, how many mail-relays I can use. I want to have 1 smtp host (1GB, Single-Core) which forwards the e-mails to another host. this host, based on the Postfix rules is keeping the e-mails in local mail-storage (Dovecot POP3) or is sending to a 3rd server with Exchange. Is that possible or is there a limitation in the amount of mail-forwarders?
You can have as many mail relays as you want.
But SMTP relays don't deal with mail storage (other than to deliver the message), that is POP3 or IMAP.
So if you really wanted to, you could have chain of a dozen SMTP relays (don't know why you would want to), and each one could decide if the message should be sent to another relay, or passed to Dovecot or Exchange for storage.
You could also then download the message from Dovecot or Exchange, and send the message on to more SMTP relays if you like.