25,465,587 - SMTP/s

Nmap

nmap -p25 --script smtp-commands $IP
nmap -p25 --script smtp-open-relay $IP -v

Connect

nc -vn <IP> 25
telnet <IP> 25

OpenSSL

openssl s_client -crlf -connect smtp.mailgun.org:465 #SSL/TLS without starttls command
openssl s_client -starttls smtp -crlf -connect smtp.mailgun.org:587

sudo swaks -t [e-mail] --from [e-mail] --attach @config.Library-ms --server 192.168.171.189 --body @body.txt --header "Subject: Staging Script" --suppress-data -ap

Reference: https://book.hacktricks.xyz/network-services-pentesting/pentesting-smtp