Log Message:
The msmtp manual states that it uses the same exit codes as sendmail.
Most of these exit codes are defined in gnulib/sysexit_.h
EX_OK is defined as 0 in src/msmtp.c if it wasn't already defined.
The variable error_code is assigned these exit values, and returned at
the end of main().

On Interix, EX_OK is defined in unistd.h:
#define EX_OK           0x8001  /* test for regular executable */

patch-ab changes the logic so that when error_code == EX_OK, the exit
code is 0 instead of EX_OK.  This should work everywhere.