Remove custom Mail::Sendmail.popen function:

  This method seems to have been added to return the output of the
standard error stream after a call to #deliver!, but it won't, so we
should use IO.popen directly.
Remove call to IO#flush in Mail::Sendmail.call:

  When a block is given to IO.popen, pipe is closed after the block and
closing the pipe flushes any pending writes, no need to call IO#flush
explicitly.