> > Your  message contains  a  BASE64 encoded  composite MIME  part,  which is  not
> > allowed by Internet standards (see RFC2045, section 6.4, paragraph 5). In plain
> > English,  an error  in your  mail program,  or in  a firewall  or mail  gateway
> > between your computer and  LISTSERV, has caused an error in  the format of your
> > message.

This error could also happen semi-incorrectly with Outlook (not sure which versions exactly). Outlook will sometimes change a message/rfc822 part into an  attached .EML file with the exact same contents, and it will sometimes use base64 encoding for that attachment. This should have been allowed, but wasn't because LISTSERV considered these .EML attachments as message/rfc822 parts, which they are in practice but not formally. This has been fixed in 15.5.

  Eric