Answer
When an client wishes to submit the body of an e-mail message consisting of a MIME
encoded content, the EHLO command is first issued to the SMTP server. If the response is code 250 and the server includes the 8BITMIME extension, the server is indicating that it supports the extended MAIL command and will accept MIME messages.
If the server does not support the 8-bit MIME extension then the client must not, under any circumstances, attempt to transfer a content which contains characters outside the US ASCII octet range.
Please see RFC1426 for further information.
Keywords:esmtp 8bitmime extension enhancement