In Outlook 2003, I have about a dozen different POP3 accounts and I am attached to an Exchange server.
When I forward emails, they often change the "from" account to a different account than the one that the message was sent to. If I "reply" to the message, the account is correct. This only happens when forwarding. Also, the incorrect account that shows up in the forwarding is not the default account or even always the same account.
So, assuming I have accounts firstname.lastname@example.org, email@example.com, and firstname.lastname@example.org. I get an email addressed to email@example.com. I hit reply, and the "From" field shows firstname.lastname@example.org. Everything is good here. But if I hit forward, the "From" field shows email@example.com (which often causes relaying rejections from my server). What's up with this?