Open up your email client (such as outlook). If you don't use your email client, then the service won't have anything to do, therefor it closes.
no it doesn't 
although the mail service is set to manual, as well as the web shield service, both depend on avastsvc main service (set to auto) and are started as a result each time you start your system. If the mail shield service has stopped, either it's been done intentionally, either there's a problem.
I know that the actual "System service" is set to manual, and will run without having a mail client, I was expecting that the OP didn't know what a "service" was, or how to get to them. I thought they were speaking of the "Provider" that shows up when you mouse over the avast icon in the system tray. When outlook is closed, it will say 8 providers total, 7 running. When outlook is opened, it will say 8 running (depending of course on how many providers they have installed).
That's the "service" I thought they were referring to.
