matthew, 
I've been doing more research on email servers and email hosting.
It would be great to talk with a Sys Admin who handles email for multiple domains and clients.
I am working with a company who has been doing web design and hosting for their web design clients since the '90s.
They hosted their email starting way back when and have been using the same email server pretty much ever since. It needs to be replaced soon.
The question is what to replace it with.
Host our own? And all the complications that come with that.
Manage the email but host somewhere else with a company that specializes in email? Such as MXroute:
Host our own email IMAP accounts, but use a separate service for sending mail so we don't have to worry about deliver-ability?
Let me know if you have experience with a similar situation.