We use Mandrill to send transactional emails and newsletters. We've just moved from SendGrid as Mandrill has far better support for templates and also integrates really nicely with MailChimp where we can design the emails.
As we started on Windows Phone we use C# and .NET for most of our apps and sites. Being a small team of 2, using .NET enables us to work on each part of the service without having to switch to different languages.
Xamarin enables us to develop for 3 platforms (iOS, Android and Windows Phone) with one core codebase coded in C#. Xamarin has allowed us to release an app on all three platforms and develop them simultaneously - not bad for a team of 2!