This is a Windows system installed file with Windows File Protection (WFP) enabled.
There are 43 versions of sendmail.dll in the wild, the latest version being 6.1.7600.16385 (win7_rtm.090713-1255). The average file size is about 56.62 KB.
(Note, the behaviors below are for all versions of sendmail.dll, select a unique version for details.)
Located in the registry at 'SOFTWARE\Microsoft\Windows\CurrentVersion\Shell Extensions\Approved'
United States installs about 48.15% of Send Mail.