Try the following varioation to see if it works:
O ClientPortOptions=Family=inet, Address=xx.xx.xx.227, Modifiers=bh
Now what I mean with conflict is that my ISP cannot have a record for
mail.orco.gr pointing to IP xxx.xxx.xxx.227 and another one
mail.orco.gr pointing to IP xxx.xxx.xxx.237.
Well, he can but it's useless here (this is used for a round robin setup).
What you need is that the IP resolves to a valid hostname. You can have xx.xx.xx.227 PTR pointing to mail.orco.gr and xx.xx.xx.237 PTR pointing again to mail.orco.gr. Or you can use a different name if you want(xx.xx.xx.237 PTR mail2.orco.gr).
As for the rest of your questions:
1. I don't think it's a routing problem.
2. You can use your ISP's mailserver (mailgate.forthnet.gr) as a smarthost without any extra cost.
3. I don't know the kind of connection you use, but I was working in a company using a leased line provided by your ISP and our /27 subnet delegation was free. You better contact them for details.