XFOR: Using Imcext.dll May Incorrectly Format Message Headers

When you use the IMC Sample Extension DLL (Imcext.dll) included in the Microsoft Exchange Server 4.0 Technical Resource Kit to reroute messages by way of the Internet Mail Connector (IMC), messages to some domains may fail, generating errors in the SMTP protocol log. Depending on the mailer, the result error codes may be:

RFC 821 states that all envelope information should be enclosed in angle brackets. When you use the Imcext.dll in conjunction with the IMC, these brackets are stripped from the RCPT TO field. This is not a problem with the majority of SMTP mailers, but some later implementations do not allow this.

Microsoft has confirmed this to be a problem with the IMC Sample Extension DLL included in the Microsoft Exchange Server 4.0 Technical Resource Kit. We are researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.