Hi Marek,
Do you mean that you want to do SMTP matching to match on-premises user accounts to Office 365 accounts for directory synchronization? If yes, have you referred to the following link to do SMTP matching: http://support.microsoft.com/kb/2641663?
As a tip, there is no Microsoft documents mentioned that we can use directory synchronization with SBS 2003 server, so we can provide limitation information about it. We are so sorry for that. But other community members who experienced the similar situation are welcome to share the information with you. Thanks for your understanding.
Best Regards,
Anna Shi