global $g_phpMailer is not set, so a new PHPMailer() is created for each message, causing a new smtp login for each message, many rapid logins can upset a mail server.
If outgoing mail queue growth exceeds mail sent between cooldown periods - you will have a problem.

If not using cron to trigger send: anything that triggers a send will cause the app to hang for all users, and possibly timeout (while it tries to send all the backed up mail).

Other than delayed (or no) mail, there is no indication of an error in the log unless SMTPDebug>0 and Debugoutput is set to a function that calls log_event() with a log level that is being logged.

--Possible Fix:
mantis/core/email_api.php

function email_send() {
  if( is_null( $g_phpMailer ) ) {
...
#line 1286
<   $t_mail = new PHPMailer( true );
>   $t_mail = $g_phpMailer = new PHPMailer( true );
...
  }
}