cat email.txt
  | { tee >(sed -ne '1,/^$/p' | process_header >&3)
  | sed -e '1,/^$/d' | process_body; } 3>&1
| sendmail -oi -- test@example.org

cat email.txt
  | { tee >(sed -ne '1,/^$/p' >&3; : branch off the header)
  | sed -e '1,/^$/d'
    | tee >(wc -l >~/.fifo); : number of body lines into pipe; } 3>&1
| xargs -I% -a ~/.fifo sed -e '1,/^$/{/^Subject:/Is/$/ (%)/}'
| sendmail ...

cat email.txt
  | { tee >(sed -ne '1,/^$/p' >&3)
  | sed -e '1,/^$/d' | tee >(wc -l >&4); } 3>&1
| xargs -I% -a /dev/fd/4 sed -e '1,/^$/{/^Subject:/Is/$/ (%)/}
| sendmail ...

xargs: Cannot open input file ‘/dev/fd/4’: No such file or directory
bash: 4: Bad file descriptor

nol="$(sed -e '1,/^$/d' email.txt | wc -l)"
sed -e "1,/^$/{/^Subject:/Is/$/ ($nol)/}" email.txt | sendmail ...