gistfile1.txt

notmuch search --format=text0 --output=files listunsubscribe:mailto  | xargs -n1 -0 awk -F: '                                                              ~
  # Set "unsub" to 0 indicating we have not hit an unsubscribe header
  # As soon as we reach one, we set it to 1 to indicate the next line
  # might be a header-folded unsubscribe line continuation
  begin { unsub=0 }

  # Abort if we find a line with no content, indicating going from
  # headers to body
  $0 == "" { exit }

  # If unsub=1 it means the previous line was a list-unsubscribe
  # header line. If the current line starts with a space, it is a
  # header continuation, so print it.
  $0 ~ /^\s+/ && unsub==1 { print $0 }

  # If unsub=1 it means the previous line was a list-unsubscribe
  # header line. Since the current line does not start with a space,
  # it is not a header continuation, so weve finished finding the
  # unsubscribe header.
  $0 !~ /^\s+/ && unsub==1 { exit }

  # We have reached a list-unsubscribe header, so set unsub=1 to
  # handle a header-folded line continuation and print the header
  tolower($1) == "list-unsubscribe" { unsub=1; print $0; }

' | sed -e 's/^List-Unsubscribe://' \
| tr ',' $'\n' \
| sed -e 's/^\s*//' -e 's/\s*$//' -e 's/^<//' -e 's/>$//' \
| grep -e "^mailto:" \
| sed -e "s/^mailto://"