Error 2603 in O365 can be resolved by following these steps1234:

  • Repair your account in Outlook by choosing Account Settings > Account Settings > Email tab > Repair.
  • Close OneDrive and all Office apps. Open Control Panel > Network and Internet > Internet Options > Advanced tab > scroll down and find TLS 1.0, TLS1.1, TLS1.2 > make sure they are all selected > Apply and OK > Restart your computer.
  • Click start, type Run and click on the run box > Paste this in: RunDll32.exe InetCpl.cpl,ResetIEtoDefaults > Click the Reset button and reboot.
  • Update Windows.
  • Check whether you're behind a proxy server.
  • Check whether you're behind a firewall.
  • Check BrokerPlugin process.
  • Check external DNS for incorrect CNAME records.