OK, now explain where you have the default settings configured. You can have
a default account on Tools>Account Settings>E-Mail Accounts and you can have a
default delivery location on Tools>Account Settings>Data Files. The account
that's chosen for the default will be the account that sends the message when
it is a new message. However, when you reply, the account used to send the
message will be the account that received the original message no matter which
account is the default, unless you specifically choose the account with the
Accounts button.
Are you saying, then, that you have chosen a POP account as the default and
that when you create a new message, it gets sent using the IMAP account?