In order to set SMTP Authentication one can complete the task from Forums administrative section and perform below mentioned steps:
1) Login into vBulletin forums administrative section.
2) Go to vBulletin Options -> Email options.
3) Enter below mentioned details:
- SMTP Email
- SMTP Host
- SMTP Port
- SMTP username
- SMTP Password
- SMTP Secure Connection
Note: This setting does not work when PHP SAFE MODE is enabled.