Hi
For ythose of you using Google Apps you will need to set your 3CX server to send to one of the MX servers for google. eg: aspmx3.googlemail.com . You do not need to specify credentials for the SMTP connection as long as the domain part of the 'reply address' is the same as the domain part of the destination address.
Any SMTP server that allows a MAIL FROM containing yourdomain.com will be able to process the emails from the 3CX server. If you are running a mail server inhouse point 3CX to it and avoid sending the emails out to an external source.
If you have issues with SMTP based issues, you shoudl recheck the SMTP settings and enable the 'notify user when extension created', then create a test extesnion and see if the mail is received. Furthermore ensure that MAV and MP3 files are not blocked by a corportate email filter of some type.
For those running non port 25 mail servers, please confirm if the mail server is seeing the connection.