[dba-Tech] email issue

I have an a situation where a client receives incoming email from an
(affiliated) corporate account but the corporation does not allow her to
send email back through that account.
 
I set her up with another email account to use for replying with.
 
In Outlook Express she has to always check the from field to make sure that
it is from her own account or the sending operation will fail.
 
She's not too computer friendly to start with and she is quite frustrated
with this arrangement.
 
Does anyone know how to make Outlook express always send email from a
specific account (even if the reply is in regards to a message that came via
a different account)?
 
If not does anyone know of an email client that can do this?
 
TIA,
John B.