My server, which is Java based, is tightly integrated with MailChimp for all my mailing purposes. The opt-in form is embedded in my desktop software and filled when the user activates my software. The details are sent to my server which sends them to the MailChimp server. The user immediately receives a welcoming email using a transactional campaign. The data stored in MailChimp includes expiration date, allowing me to send reminder emails to my users using autoresponders.
Coding my integration took more time than I expected. It was a lengthy process of trial and error, figuring out the fine details. Currently, the MailChimp API officially documents the integration with PHP, .Net and Ruby on Rails. Java is an uncharted territory. Eventually, I got my code running smoothly. After understanding the basics, moving forward is an easy task.
The good news: I'm here to help.
I'm in the process of packaging my code and releasing it for public use. I will be happy to assist in integrating your Java software with MailChimp. My company is keeping me very busy, so I am not looking for full integration projects. I am available for private consultation sessions on per-hour basis using phone, email or online chat (can meet if you are in Israel or willing to pay for my travel :-).
Send me an email to zviki [at] nwiresoftware.com for further details.