.PS
# in and out channels of an mta

linewid = 1.5


A: box "MTA" wid 1.5*boxwid ht 1.5*boxht

line <- left from 0.3<A.w,A.nw> "remote (SMTP)" above
line <- left from 0.3<A.w,A.sw> "local (stdin)" below

line -> right from 0.3<A.e,A.ne> "remote (SMTP)" above
line  right linewid*2/3 from 0.3<A.e,A.se> "local (pipe)" below
line -> down 1/6*linewid
B: box "MDA" wid 0.75*boxwid ht 0.75*boxht

move 0.5*moveht

C: box invis "mailboxes" ht 0.3

arrow from 0.35<B.sw,B.se> to 0.2<C.nw,C.ne>
arrow from 0.45<B.sw,B.se> to 0.4<C.nw,C.ne>
arrow from 0.55<B.sw,B.se> to 0.6<C.nw,C.ne>
arrow from 0.65<B.sw,B.se> to 0.8<C.nw,C.ne>