Hello ToNel,
also in a small organisation i think it is very important to chose the right product for your needs.. in my eyes and this is no bashing against Wingate ít is better to use a more specific product for your needs. Wingate is good as a internet gateway and using as a VPN Solution.. yes also Mail is possible but did you think already about the spam problem. So you will not come arround to use additional products ... ok you can use a anti spam solution on the client side if you use a mail client to recieve the mail from wingate.. but anyway i think you should take a look to the Hexamail E-Mail Server
http://www.hexamail.com/hexamailserver/You can add a very good anti spam module and some other function you need at anytime... also a out of office responder
We are using the Anti Spam Module since 3 years and we are very very happy with it...
Greetings