I have a website for newsletters, and I'd like to give my customers the possibility to preview how the message will be displayed on Outlook (various versions), Thunderbird, LotusNotes, GMail, etc...
I know there are many websites and applications out there that do this trick, but I want to implement this feature inside my website.
The newsletter website is written in PHP on a Linux standalone server which I have total control on.
There are 2 main services that provide email testing. Litmus and Email on Acid.
Email is very hard to accurately simulate across email clients, and I wouldn't suggest trying to do it outside of a service that has virtual machines running the tests for you.
Take a look at the API's (Litmus and EOA) of each of these 2 services. Email on Acid seems most cost effective option. Using the API of one of these two services is the only (accurate) option I would suggest.