Although Open Office is the office software that everyone knows off of the top of their heads, it's not the only great office software in existence. StarOffice is another popular office software suite that delivers on a lot of levels. Is it as popular as Open Office? Not really, but it has a dedicated userbase all its own. If you're not a big fan of Open Office or you just want to try something different, you may want to give StarOffice a chance and see if it's more your speed.
SDM - StarOffice Mail Message refers to the actual email files exchanges within the StarOffice system. Note that these files are not compatible with Open Office, so if you use StarOffice, you'll want to keep it as a solution to your entire office's needs rather than mixing and matching different types of office software.
If you have a home office, of course, then it should be no problem as you can simply pick and choose an office software package at your leisure.
If you're still deciding on office software, give StarOffice a try and see if it suits your needs. There are a lot more options out there than just Open Office.