To stop guessing, I downloaded and installed trial version of Office 2007 and connected it to the Exchange server. Configured exchange as a main account and added POP3 account, delivering mail to Inbox folder created in local PST file. OF course all works as expected so speculations it is not possible send mail using POP3 acount if another MAPI account exist is false.
Sorry for czech version of apps but I am sure you will undersand the important parts:
First - two accounts in one Outlook 2007 profile, first is Exchange server, second is my private POP3 account on freemail
Creating message and selecting POP3 account as sender of the message:
When I was sending this mail, I was asked about SMTP server password so I knew Outlook is using SMTP server provided in details of my POP3 account and not the Exchange server for sending outgoing mail.
Of course when it arrived it was sent from my POP3 account:
What more do you think I should do to persuade Scalix it is bug of their Connector and not Outlook "feature"?