When an MTA SDK routine is called by a processing thread and returns an error status code, the SDK saves that status code in thread-specific data. The same processing thread can obtain the most recently saved status code for its own thread of execution by calling mtaErrno().
For convenience purposes, the mtasdk.h header file also defines mta_errno as a macro that calls mtaErrno(). Specifically:
#define mta_errno mtaErrno()