Kristi - what issues do you find with Outlook? I'm curious because I receive well in excess of 150 emails per day and have to say it works fine
Personally, I'd say the most important thing is to make sure you have rules set up for each mail account/client. Have folders in place for each client and subfolders for each assignment. Read & action each mail immediately - don't just skim through it and make a mental note to deal with it later; something more important may come up and it'll get overlooked. Make use of the flags to highlight Priority 1, 2, 3 tasks etc.
Also, if you receive a high volume of emails, one thing that will save your sanity is to turn off the auto-receive feature in Outlook - how many of us can ignore that 'ping' to let us know we have new mail? Use the time when you come to a natural break between tasks to check your mail and prioritise/action it.