Microsoft essentials not updating
There is a definition update for MSE listed twice – on build 1.249.1294.0 and on 22.214.171.124.
Entering the update history, there are several failed entries (see the screenshot below).
But some users are facing the scenario shown in the above screenshot.
Microsoft Security Essentials and Windows Defender are free antivirus product for Personally Owned computers and organizations of 10 or fewer computers.
Microsoft Security Essentials is for Windows Vista and Windows 7 computers while Windows Defender is for Windows 8 and Windows 10 computers.
Addendum: I found an Microsoft Answers forum thread Microsoft Security Essentials Update Definition Number 126.96.36.199 Keeps On Coming Back, where another user has documented this issues (with screenshots from an English Windows 7 with MSE).
In most cases, Microsoft suggests to uninstall Microsoft Security Essentials (MSE) und reinstall MSE again.