To sort out outlook, disable cached/offline mode in outlook 2007.
Regarding apple-Mail:
I experience sometimes a thing, that, If I write a new message, a copy of the Message will be saved in the Drafts folder, even, If I had sucessfully sent it.
Also, sometimes, When I send a Message with apple Mail, the mail will be tagged as "unread" in the sent items folder.
If the inbox is large in apple mail (>about 4000 items), apple mail might overlook some new emails, it even might hide some messages which it had seen previously. However quitting the application and starting it will make it fetching the Messages again (but, this occurs AFAIK only in 10.5.x apple mail).
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Email clients are getting worse today, because many people read their mail with the browser. So, Developers do not spend much time developing nice email-clients. Still Apple-Mail is one of the better ones.
See here:
Mozilla Thunderturkey and its malcontents âxxx8364;¢ The Register
for a interesting rant about this topic.