I thought that exim is associated with a smtp server.
If that is true double check your mail setting.
Older versions of ubb allowed you to set the smtp or sendmail path.
The newer versions only allow you to set the smtp path.
So check:
Control Panel » Primary Settings>General + Advanced Options
Use specified SMTP Server:
Make sure you don't have anything in the field.
If you are using sendmail this field should be blank.
Maybe your host allowed smtp for some time and now does not allow it.
You might want to also check:
» Primary Settings>General
Board Email Address:
And make sure it is a valid address from your host server. Not something like a aol or hotmail address which would not be recognized coming from your server.
