Com awk :
nmap --script smtp-open-relay.nse --open -p 25,2525,587,465,2526 -iL ./short_servers_list.txt \
| awk '/^ *Nmap scan report/{ sub(/^ *Nmap scan report for */, "", $0); printf "%s ",$0 }
/_smtp-open-relay/{ sub(/^ *\|_/, "", $0); print $0 }'