It is a job and a half to run your email service (!).
There is one good solution out there at Everyone.net - they run a webmail service on your domain, but using their servers.
Since they are a massive operation, they take care of all the necessary maintenance etc., and you just administer the settings like the appearances of the mail accounts, and the users etc.
They used to have a free option, but now it is a monthly-paid-for service, however it isn't that expensive compared to how much more professional it can make your sites look.
I use the service on my
www.Atmail.co.uk webmail service. I have about 350 users just from type-ins.