You can use a Active Directory services and logon both in the AD.

It’s simple to create the same user on windows and the NAS. My question is how can you change the Log On user for the SmarterMail service without having to restart the service? Will AD solve this?

If you change the service account, the service has to be restarted to run under the new identity regardless of whether you are domain joined or not.

I want to back the original poster on this. We were told years ago that there were plans to implement something to help aleviate this. Sometimes my failover.json file needs to be fixed, and Nearly every time I update smartermail the service either fails to start and the upgrade process fails to go through unless If I leave the error screen up, fix the credentials on the service, and tell it to retry or the install goes through, it replaces the failover.json and starts a server that has no data tied to it--In which case I need to shut down the server, shut down IIS, replace the Json file, and update the credentials tied to the service before starting up the shared IP, IIS, and the smartermail service. Its honestly a royal pain and probably the most offputting thing about smartermail at the moment. 

We have similar issues where JSON files are going blank without any explanation. Weird issues with upgrades and downgrades and I have a feeling it is related to this. It's just those 5-10 minutes during the install where thousands of users are kicked out of mail clients because Smartermail cannot access the storage unless the service is restarted with the new identity. 

We have another application where it can access UNC paths with a user name and password that is specified in the GUI. Something like this for Smartermail would be great. 

This also pops up quite often, delaying the restart process even further.