sendmail, 4th Edition

Basic Textual Canonicalization

Addresses can be legally expressed in a variety of formats:

address
address (full name)
<address>
full name <address>
list:members;

When sendmail preprocesses an address that is in the third and forth formats, it needs to find the address inside an arbitrarily deep nesting of angle braces. For example, where is the address in all this?^[261]

Full Name <x12<@zy<alt=[e-mail]<[e-mail]>r.r.net>#5>+>

The rules in a typical canonify rule set 3 will quickly cut through all this and focus on the actual address:

R $*                     $: < $1 >                       housekeeping <>
R $+ < $* >                 < $2 >                       strip excess on left
R < $* > $+                 < $1 >                       strip excess on right

Here, the first rule puts angle braces around everything so that the next two rules will still work, even if the original address had no angle braces. The second rule essentially looks for the leftmost < character and throws away everything to the left of that. Because rules are recursive, it does that until there is only one < left. The third rule completes the process by looking for the rightmost > and discarding everything after that.

You can witness this process by running sendmail in -bt rule-testing mode, using something such as the following. Note that some of the lines that sendmail outputs are wrapped to fit the page:

% /usr/sbin/sendmail -bt
ADDRESS TEST MODE (ruleset 3 NOT automatically invoked)
Enter <ruleset> <address>
> -d21.12
> canonify Full Name <x12<@zy<alt=[e-mail]<[e-mail]>#5>+> >
... some other rules here -----trying ...