smtp mailfrom meta-syntax issues

Folks,

there has been discussion about meta-syntax restrictions in the
rfc2821.mailfrom field.  I am forgetting what restrictions were cited.
That is, what will break existing MTAs?  Which characters and what
string sizes?

Thanks.

d/
--
 Dave Crocker <mailto:dcrocker(_at_)brandenburg(_dot_)com>
 Brandenburg InternetWorking <http://www.brandenburg.com>
 Sunnyvale, CA  USA <tel:+1.408.246.8253>, <fax:+1.866.358.5301>