In the SMTP protocol, a blank sender email address has a special meaning. It means that you do not want to receive bounce messages. This is useful with automated messages. For instance, you do not want to receive bounce messages in response to another bounce message - or you could end up with a loop with two mail servers endlessly sending messages back and forth to each other. Similarly, receiving bounce messages to autoresponse messages etc is also generally undesirable. In these cases, the Standard way to prevent the bounce messages is to use a blank sender address (also known as a null return path, or empty return address or similar). The SMTP standard requires that the null return path is accepted and processed correctly.
Unfortunately, some Internet Providers have decided that the email standards do not apply to them, so have configured their servers not to accept a null return path, even though there is no security risk in doing so. (It is trivial to forge the return path, so relying on it for anything to do with security is pointless).
These Internet providers may refuse to accept messages from mail server autoresponders or bounce messages.
To get around this problem you have to tell VPOP3 not to work correctly, and not to use null return paths in the places it should be doing. To do this, go to Mail Connectors → Mail Senders → Return Path Settings. Set the Change NULL return addresses to setting to another address.
You have to be very careful what address you set this to. It must be a valid email address which will never have an autoresponder, forward or assistant defined. If you don't do this correctly, then you risk setting up an infinite loop of error messages between your VPOP3 server and another mail server.
(Ideally, you should complain to your ISP and get them to change their policy on null return paths, since there is absolutely no benefit to them blocking these messages).