Outlook’s junk mail system is fairly basic and, if you receive more than a couple of emails from the same source (and particularly those from a .com email address), Outlook can quickly regard these as a spam and filter those on your behalf. If you’re anything like me, I rarely check through my junk mail folder. Worse, if someone from this email domain emails you to ask if you received their previous email, you may find that is automatically classed as junk, too.
Ideally you’d turn off the Outlook junk mail filter and use something different. One option is the SpamBully anti-spam filter that enables you to determine what you class as spam or legitimate. If you want to read your email on a mobile source, you won’t want any spam at all, so SpamBully will only make legitimate email available for reading through your phone or other device.
Note that there is also an Outlook Express and Windows Mail edition.