Multipurpose Internet Mail Extensions , or MIME is a standard format for email messages.
RFC822 items can be big, and correspondingly expensive to manipulate. tcllib's mime package parses any presented item completely. What if one only wants, say, the "Subject:" of the item, while needing performance? In principle, there ought to be a "lazy" evaluation that intelligently scans the few header lines necessary to determine this value.
A comp.lang.tcl thread [anyone have a reference here?] discusses this. One highlight is that Don Libes's "mail notification tools (i.e., biff)" page on tkbiff points to code which directly models such a requirement.
AMG: MIME is also the Militant Internet Mail Enthusiasts [L1 ]. This is apparently just a humorous page with technical resources for email such as links to (older) RFCs.