Well it wouldn't surprise me too much if it was outlook. It is a truly appalling program. For example, every few days, my favourite folders disappear. Every few weeks, one of my IMAP accounts reports that the folders are no longer accessible - I can't remove the folder, all I can do is remove the account and recreate it and forget about the old folder! And then every so often emails appear to be duplicated, and then outlook goes off and re-scans the imap mailbox and returns with either the same duplicate items, or on rare occasions, it displays the emails properly. Sometimes, attachments get duplicated 8 or so times!
So I think in this case it's probably outlook that isn't working properly