A little more information would sure help.. Operating system on the machine? Make and model? EXACTLY what was done when the motherboard was replaced?
As long as the motherboard is the same model as the previous one, and you didn't reformat the hard drive, the computer should run as it did before... Everything... The operating system, the programs, your documents, and your email should still be there.
Or was the hard drive formatted and everything reinstalled from scratch? If that's the case, then all data on the drive is gone..
Or was a new hard drive installed on the machine and the old hard drive set aside.. If that's the case, then the old hard drive should still contain your messages and they could be recovered using a number of different methods, such as placing it in a USB enclosure and using it like an external drive....or maybe just adding it as a slave drive on the current computer.
Hope this helps and let us know more.
I have just had a new motherboard installed and wondered if I could retrieve previous emails etc from Outlook Express, I have obviously had to set it up again but cant work out if I can retrieve previous stuff.