Sounds to me like you're looking for an application with the following:
- Email integration
- Setting tasks/to-dos
- Help desk support features
There is a variety of different programs out there, and it really depends on what you level of business management software you want to incorporate into your business. As Matpro said, there is VisionProject, and then popular project management tools like Basecamp.
You may find that after you have integrated one feature you will see there are other areas you can apply business management software to. Many come to this point after seeing the power of these useful systems but eventually have a variety of different applications that don't integrate well with each other and this only results in a headache. If you want to avoid this, WORKetc may be what you are looking for - or it may not be, you need to look and see for yourself. The WORKetc application incorporates project management, crm, billing, and help desk features into one system. It's essentially a total business management tool, which allows a much higher level of collaboration (you can collaborate on everything) and employee communication. It's a lot simpler to use to send reminders, make sure emails are being followed up on (you can see if they are), and set tasks/to-dos that employees can't miss.
The choice is up to you to do your own research in these programs and find what will specifically meet the requirements of your business. Every business has specific needs and that's what makes finding these programs so darn hard!