outlook does not use default account

[LSCR]

When replying to emails, I've noticed that Outlook 2003 does not use either
the default account or the account the sender sent to.

I have three accounts: a.com, b.com, and c.com. b.com is my default. Even
when I am replying to an email sent to me at b.com Outlook uses c.com as the
From address. To use b.com as the From address, I have to select it manually
from the dropdown From list. When writing a new email, Outlook does use the
default address.

Any ideas on how to fix this?