TIP
When performing username enumeration use the following username wordlists, starting small (for quick wins) and gradually expanding - add custom names to the lists (if appropriate):
/usr/share/wordlists/metasploit/namelist.txt(small)seclists/Usersnames/Names/names.txt(med)seclists/Usernames/xato-net-10-million-usernames.txt(big)
sudo nmap -p 25 --script=smtp-enum-users $trgt1
If you have a username, try mutations of the username against services 2.0-Enum Hints and Misc
smtp-user-enum -U /usr/share/seclists/Usernames/xato-net-10-million-usernames.txt -t $trgt1
user enum
smtp