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Error: Why does CDO.Message give me 8004020F errors?

The e-mail was rejected by the server for some reason. Here are some things you can try to alleviate the problem: 
  1. Make sure the SMTP server allows anonymous (non-authenticated) relaying. If your SMTP requires outgoing authentication, see Article #2026
  2. Check if the problem is specific to the domain name(s) used in the e-mail addresses of the recipients. For example, some users have complained that they can send to users on their own domain only; others have said that they can send to any domain except their own (see Article #2511 for some potential causes and solutions). 
  3. It may be simply that the e-mail address is being rejected, but other configuration settings on the SMTP server are preventing the true error message from being relayed propely back to the ASP script ... so verify that the address is valid. 
  4. If you have a proxy or firewall, make sure the web server is set up to correctly pass through it, that the SMTP server knows about it, and that the proxy allows access to port 25. 
  5. Try using a SendUsing value of 1 (pickup) instead of 2 (port). E.g. the following line: 
    .Item(cdoSendUsingMethod) = cdoSendUsingPort
    .Item(cdoSendUsingMethod) = cdoSendUsingPickup