Outlook 2003 receives email at one address but reply from another

[bwana]

I have the same problem discussed in a previous post but it is not clear what
the solution is.

I have several email accounts retrieved by Outlook 2003.

Quite often, when I receive email sent to account abc@domain1 and reply to
it, the reply goes from account def@domain1.

Today, the reply went from account ghi@domain1 -- that is a third account.

I have not had a reply go from either account jkl@domain2 or mno@domain2
when replying to an email from domain1 and indeed, am not aware that this
problem occurs with domain2 mail.

Any thoughts?

Thanks.