If the 'explain' directive is going to remain in the SPF spec, I would
suggest that some language be added to encourage (require?) administrators
to limit the explanation to the ASCII character set and to keep the
explanation short enough so that - after macro expansion - the explanation
will fit on a single reply line in the SMTP protocol.
The maximum total length of a reply line including the reply code and the
<CRLF> is 512 characters, as per RFC 2818 section 4.5.3.1.
Daryl Odnert
Tumbleweed Communications
Redwood City, California