Repair a broken scheduled mailing system

Needs: * PHP * MYSQL * Mail sending software experience * Cron experience Problem: I have a custom made scheduled mailing system that was working fine for 4 years. I moved my server, from old Ubunto to CentOS and it stopped working. -> problem with sending queue, server seems to get overwhelmed though it is way more powerful than previous server. -> problem with registration system (system check from registration mail on some predefined addresses with IMAP, and register new email addresses from inbox emails). This work should be really easy for someone who is at ease with those technologies. It was working before, I just want you to understand what is wrong now, and what is to be change. Shouldn't be more than some modifications. I will give much more explanations afterwards or on demand. This system is a running system on a production server, so I can't afford mistakes. Regards, Rui