The primary reason for mail not being forwarded is if it is being classed as spam. If an email has been designated as spam, it may be received by one email address but then not forwarded-on to the subsequent email address. This helps the reputation of the server and prevents blacklisting.
To resolve this, you can try lowering the spam filter via Junk Filters within the Arahoster control panel.
To do this:
- Login to Control Panel
- Locate the package containing the forwarder and select Junk Filters
- You could also try whitelisting the domain/email address the email is coming from.