[exim] Possibly OT

New to Exim and using Exim 4.43 and Dovecot 1.0.13 on CentOS 4.6. Server
sends and receives mail but when sending from Squirrelmail it is sent as
user 'apache@???' and because of this replies are rejected with a
'550 relay not permitted (in reply to RCPT TO command)'
Any ides on this one? Used to work normally with Postfix in place of Exim
on same server.

regards,
Chas.