The SMS Server Network Connection account (SMSServer_sitecode) is used by:
SMS Setup creates this account on the site server and adds it to the Domain Users group in the site server's domain. The account has Read permission on the Sms directory, Full Control permission on the SMS_SITE share on the site server, and permission to write DDRs.
You can create multiple accounts on logon points, and SMS will try each in turn until the connection is successful. Create an account for each domain in the site.
Specify a New SMS Server Network Connection Account
SMS Accounts Configuration Overview