outlook 2003 ,can set default sender account?

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i have outlook 2003 whit 1 pop3 account and 1 exchange account .
if 1 email is send from a internal domain user and wanna to reply outlook
wanna send message whit exchange account (and if im in travel "dont work"),
me i wanna send all mail whit pop3 account., and all the time need change
this whit the account button...

this appen only if i use the reply button, in new message outlook use the
default account and work fine.

any idea to make the pop3 account the DEFAULT Also in reply mode?

tnx all