How to config the "send from" field?

[York College]

Hi,

I have php 4.44, Horde 2.0 and imp 3.0 runing on solaris 8. When I
log in to our email server from imp and compose an email, the From
field has "Default". 

Does any one know how and with which config file I can change the default
to the address I designated. I know I can write it in the "or" field
next to the "Default" in the compose window. But I'd like to do it
 for everybody using this email server and imp.

Any hint will be very much appreciated.

Laura