Server Email Issues / Resolve - External Mail / SMTP Service

We are currently having issues with Hotmail & possibly other providers blocking our emails so users can not recieve any emails such as registration confirmation or password reset requests. We are looking for someone who can help resolve this issue quickly by either resolving any issues we have on the server and working to resolve on our behalf with hotmail and any other providers Or which I think would be prefered would be to move all mail functions to an external mail service. This Task May Include: Server Modification / Optimisation PHP Script (Mail Function) Modification Liasing With Mail Providers (Hotmail etc.) Setting Up / Integrating an External Mail Service. Our server is running Freebsd & has NO Cpanel installed.