By default, UnderConstructionPage can connect to MailChimp and Zapier. If you are using any other autoresponder like Aweber, GetResponse, Mad Mimi, ActiveCampaign, Campaign Monitor, etc. you can still connect them to UCP as long as those services can generate HTML forms.
In that case, you will just have to generate the HTML form and copy it to the plugin. UnderConstructionPage will take it from there and will arrange everything so that you can start collecting leads.
Settings -> UnderConstruction -> Settings -> External Services
After that, you will be able to select the universal autoresponder as an option when using contact form element.
Customers who purchased and have a valid PRO license - contact our support via the contact form built into the plugin or use the general contact form. If you're using the later method please include your license key in the message or contact us via the email address used when purchasing. This speeds things along.
Using the free version? We have you covered too. Head over to the forums. First look at older threads and if you don't find anything useful open a new thread. Support is community-powered so it can take some time to get an answer. If you need premium support this second - upgrade to PRO.