I have my Outlook 2010 storing mail in an Exchange 2007 SP3 mailbox. I have a long standing POP3 e-mail account that is set-up in Outlook along with the Exchange account, and couple of other POP3 accounts and a GMail account as well.
The long standing POP3 account is set-up as the default account in Outlook, but every time I reply to a message Outlook tries to use the Exchange account instead of the PO3 "default" account. If I create a new message from the "New E-mail" button on the Ribbon it does use the "Default" POP3 account, but if I right-click on a contact and select create E-mail it tries to use the Exchange Account again.
There is another person here who has the same set-up (POP3 as defalut, storing mail in an Exchange mailbox), who does not have these problems. We have compared each Outlook set-up and checked various settings on the Exchange Server looking for something that would make this happen to no avail.
Since we use Exchange to store our mail rather than being the main outgoing mail server, if I don't pay attention and change the send by account to the POP3 account instead of the Exchange account, then receipent mail servers that test for reverse DNS look-up routinely reject the messages.
I thought we could at least get around the reverse DNS rejects by pointing top priority on our MX record to our public IP address, and giving the ISP for the POP3 accounts secondary priority.
To make matters even more complex, the reason for changing the MX record to point at our public IP address was so that the mail would come here first, be filtered through our on premises E-mail filter appliance, then be routed to the POP3 ISP's mail server address, and finally be delivered to our Outlook clients where we have the POP3 accounts configured as the "Default: amil account.
This is caused its own set of issues, with some receipient mail servers seeing this as relaying.
However, the original problem (the "default" account getting trumped by the Exchange account) has existed long before the MX record and mail routing scheme described above, and continues now.