Timeout too short in class.phpmailer.php
In vtiger 5.4.0 the $Timeout var was set to 60 seconds in line 193:
var $Timeout = 60; // Fix for http://trac.vtiger.com/cgi-bin/trac.cgi/ticket/5389
In vtiger 6 this is set to 10 seconds on line 298:
public $Timeout = 10;
For some SMTP servers we have found this is far too short as they are configured to deliberately wait 30 or 40 seconds before responding as an anti-spam technique.