A ~/.forward file must be owned by the user or by root. If it is not, it is considered unsafe, and sendmail ignores it. The -d27.4 debugging switch causes sendmail to print a message describing any such file it finds unsafe:

include: not safe (uid=user-id)

Note that a file is considered unsafe if, among other things, it lacks all read permissions.

The -d27.4 debugging switch also causes sendmail to print information about a :include: file beyond that printed with -d27.2:

include(file)           printed with -d27.2 
   ruid=real-user-ideuid=effective-user-idprinted with -d27.4

This shows the real user-id (the ruid=) and effective user-id (the euid=) of the current running sendmail.

The -d27.4 debugging switch causes sendmail to print an error if a :include: or ~/.forward file cannot be opened for reading:

include: open: reason here