https://github.com/ruby/net-smtp/commit/6e5c09dcc4

https://github.com/ruby/net-smtp/commit/b706942392

The helo argument is not important, but the helo argument must be
specified to specify the user and secret arguments.
If helo, user, secret, and authtype arguments are keyword arguments,
it is not necessary to specify the helo argument.

https://github.com/ruby/net-smtp/commit/269774deac

In tlsconnect(), the host name is checked when
@ssl_context.verify_mode is not OpenSSL::SSL::VERIFY_NONE, but the
verify_mode of @ssl_context generated by default is nil.

https://github.com/ruby/net-smtp/commit/bde75a15b5

"requiring version.rb" strategy has some issues.

- cannot work when cross-compiling
- often introduces wrong namespace
- must know the superclasses
- costs at each runtime than at build-time

etc.

As `String#split` with the default argument drops trailing newline
as a separator, preceding `String#chomp` is futile.

This removes the security features added by $SAFE = 1, and warns for access
or modification of $SAFE from Ruby-level, as well as warning when calling
all public C functions related to $SAFE.

This modifies some internal functions that took a safe level argument
to no longer take the argument.

rb_require_safe now warns, rb_require_string has been added as a
version that takes a VALUE and does not warn.

One public C function that still takes a safe level argument and that
this doesn't warn for is rb_eval_cmd.  We may want to consider
adding an alternative method that does not take a safe level argument,
and warn for rb_eval_cmd.