PS Backup to a network file share showing up with SMB Session Failure

I have created a daily backup for my PS using Windows File shares and was pointed to one of my Windows File Share using a domain service account. This one works fine. My only concern is that we keep receiving alerts about NTLM login failure/SMB session failure for the service account since it was running from the U-series appliance (non-domain joined) and connecting to the windows file share in the domain using the domain service account. 
Even though the daily alerts keep popping up with those failures, we still have the completed backups. I was wondering if you guys have any better option with this backup and minimize these kinds of alerts?