David, there are no settings/software which are going to be able to distinguish e-mails which you send to businesses from e-mails which you sent to friends/family. Something else is going on.
First, are you thinking that because you don't get a Read Receipt that your e-mail is not being read? This is not true. The receipients have the choice as to whether to send you a receipt. They can read your e-mail without sending you a receipt.
Second, it is just possible that your firewall (certainly your anti-virus program) may block incoming e-mail traffic, but it is unusual for a firewall to block outgoing e-mail. Could your e-mails contain content which a business might block? Are you attaching large files? Are you attaching executable files (.exe)? Is your e-mail unsolicited? (Spam?) Normally if an organisation's firewall blocks an incoming e-mail the sender would receive an e-mail informing them that the e-mail has been blocked and that the receipient did not receive it. These e-mails would normally explain why the e-mail was blocked. If you are not receiving these, then I don't think your e-mail is being stopped.
Justin T
September 2003