Hi
postfix is probably the best smtp server you can have in a VPS server.
If your on ubuntu / debian
-> apt-get install postfix
if your on centos
-> yum install postfix
Then you have plenty of how-to configure SMTP relay for postfix
Configuration:
this is the config file /etc/postfix/main.cf
And find ( or add ) this entry
myorigin = FQDM
This hostname ( you VPS ) will need a rDNS entry also
If your application which send email is on the VPS then you should be fine
If your application which send email is on ip 192.168.10.20
Find this entry and add a SPACE and the IP of your sending application
-> mynetworks = 127.0.0.0/8 192.168.10.20/32
Then you should be fine.
Good luck.