Continued development on existing C# webtool (including a bit of Linux/Qmail)

This is a relatively complex website built from scratch on .net in C# (MVC) combined with MS SQL and a little jQuery, which is focussed on allowing people to register online for multiple newsletters. Subsequently, these newsletters are collected and forwarded to the actual recipient via another server (CentOs using Qmail and MySQL). The two servers of course are connected and interact. The website needs additional features and for this we look for a reliable team/partner who knows both technologies/platforms and who can help us on this projects for the years to come. We're only interested in Senior people with a proven track record in all technologies.