"Janice" wrote in message
...

All mail gets sent on the pop account that is the default (as long as I
remember to manually choose it). Replies from that account are then
forwarded onto the imap account so that I can access them from home, work,
mobile etc.

This explains it, then. You receive all mail via the IMAP account, since you
forward the mail received by the other accounts to the IMAP mailbox. Outlook
never sees messages arriving from those POP accounts, only from the IMAP
account, so all replies go back out through the IMAP account. Outlook doesn't
care one whit to which address the original message was addressed. All it
knows if what account was used to receive the message.
--
Brian Tillman [MVP-Outlook]