Project Page: http://common-lisp.net/project/cl-smtp/
See also Franz's imap which has an SMTP client.
New Version: 2010-05-05
Rewrite encoding functions, now it is possible to use non ascii characters in header values and in attachment filenames.
New keyword argument external-format (default :utf-8).
New class attachment, with slots attachment-name and attachment-mime-type, and a constructor function make-attachment.