Add domains from mail.gw

[e-nomem]

These domains are used by the temporary mail service, mail.gw. This seems to be a copy of mail.tm and we can use the same method of verification as in #465 except update the MX record we're looking for to in.mail.gw.

The following script can verify the list:

#!/usr/bin/env bash
set -euo pipefail

PR="467"
PROVIDER="mail.gw"
MX_RECORDS=("in.mail.gw.")

while read -r line; do
    echo "Checking $line"
    mx="$(dig +short MX "$line" | cut -d' ' -f2-)"
    grep_args=()
    for search_record in "${MX_RECORDS[@]}"; do
        grep_args+=('-e' "^${search_record//./\.}$")
    done
    grep -q "${grep_args[@]}" <<< "$mx" || echo "$line is not part of $PROVIDER (MX: $(paste -s -d' ' - <<< "${mx:-missing}"))"
done < <(curl -fsSL -o- https://github.com/disposable-email-domains/disposable-email-domains/pull/$PR.diff | grep '^+[^+]' | cut -d'+' -f2-)