Relying on the PHP internal mail function has always caused problems with a few configurations. The PHP internal mail function is simply a wrapper for the systems own mail program such as sendmail or procmail. The problems occur when certain configurations have extra authentication settings or confusing with line endings. This has been resolved by the introduction of our mail class to correct any problems that may occur with the internal mail function and also to allow the use of SMTP which will completely bypass PHP's own internal mail function.
The SMTP server options can be edited via the Email Options in the vBulletin Options, this will be enabled via the SMTP Email switch and then the appropriate settings must be completed.
It should be noted that the majority of servers do not require a username and password to send SMTP as they are limited by IP address. Also the use of your ISP email is not recommended as this is usually limited to the network of the ISP and will result in a failure to send email.