SMTP
Introduction
The Simple Mail Transfer Protocol (SMTP) is a protocol for sending emails in an IP network
SMTP is often combined with the IMAP or POP3 protocols, which can fetch emails and send emails.
SMTP runs on port 25 UDP by default
Newer SMTP servers also use other ports such as TCP 587
SMTP Enumeration
telnet <FQDN/IP> 25 followed by EHLO nameserver.htb or HELO nameserver.htb
Check connection to SMTP
Test-NetConnection -Port 25 192.168.50.8
Check connection to SMTP (Windows)
smtp-user-enum -M VRFY -u root -t 192.168.1.25
Enumerate SMTP user "root" using the VRFY method (if available)
sudo nmap 10.129.14.128 -p25 --script smtp-open-relay -v
SMTP Open Relay server enumeration via nmap script
SMTP Open Relay
An open relay is a SMTP server improperly configured to allow an unauthenticated email relay.
A SMTP Open Relay allows mail from any source to be transparently re-routed through the open relay server.
This behavior masks the source of the messages and makes it look like the mail originated from the open relay server.
Useful for phishing purposes
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