Hi,
1) Log into the back-end.
2) Then locate "settings" under the administration panel
3) In the first field locate: " — Enter email to receive system messages"
4) enter the email address you wish offline emails to be sent to
5) save
Try sending an offline message.
When you edit the config file ($webim_mailbox =) you are editing the FROM email address. I have tested this and proven this is the case.
Let me know how things go for you.
Hope this helps!
Gabe