[throw16180339]

It's a lot better than sendmail.cf, which I had to use in a previous job.

    # handle "from:<>" special case
    R$*<>$*                 $@@                    turn into magic token
    
    # basic textual canonicalization
    R<$*<@$+>>              $@$1<@$2>
    R$*<$+>$*               $2                     basic RFC822 parsing
    
    # make sure <@a,@b,@c:user@d> syntax is easy to parse -- undone later
    R@$+,$+:$+              @$1:$2:$3              change all "," to ":"
    R@$+:$+                 $@$>6<@$1>:$2          src route canonical
    
    R$+:$*;@$+              $@$1:$2;@$3            list syntax
    R$+@$+                  $:$1<@$2>              focus on domain
    R$+<$+@$+>              $1$2<@$3>              move gaze right
    R$+<@$+>                $@$>6$1<@$2>           already canonical