The requirements and the budget contradicts.
A mail server like ability mail server will consume atleast 1200 - 1500 man days = 77K - 96K Euros (as per the minimal pay). Your budget is 10K. Even if you had considered a low price tag on a outsourced project, its ~50K Euros. So, what i feel is that, your budget and your description ( almost all features of ability server ) doesnt match. If you can give a better requirement against the 10K budget, it will be more effective.
My Proposal:
I have 4 C++ guys with a minimum of 12 years of experience with C++. We dont have SMTP/POP3 experience, but its just a protocol, we can hanle it. We work for reputed medical equipment manufacturer and we comply to CMMI Level5 Dev 1.2 procedures; means we can offer quality by our process.
We can work on an iterative model development, which will span between 6 to 9 months (if you are to create a quality software which can be sold, please invest time also).
Current bid:
180 working days * 6 resources * 50E/day = 55 K Euro + 10 K Euro for linux support.
But we know that, this bid also is not correct. It can be corrected later, if we start working together. Otherwise, i hope that you will get an idea of the development effort required for such a software.