you can use fake sender addresses.
Sorry Falko but I can not agree with that.
I have another domain which is not on this dedicated server , but is on a shared hosting on a windows server of ixwebhosing.com .
When I am trying to send mail to one of the mailbox of this domain using a fake and non existant userid of the same domain , the mail server is not allowing me to send it.
And that is also SMTP protocol .