0.21.0

go-smtp v0.21.0

This release contains multiple breaking changes:

- Servers implementing Session.AuthPlain need to switch to AuthSession
- Server.AuthDisabled and Server.EnableAuth have been superseded by AuthSession
- Client.StartTLS has been superseded by DialStartTLS and NewClientStartTLS

IKEDA Soji (1):
      Server: The value of BODY parameter is case-insensitive

Simon Ser (18):
      Add DialStartTLS
      Unexport Client.StartTLS
      De-duplicate SendMail/SendMailTLS
      Fix tls.Config ignored in DialStartTLS
      Add NewClientStartTLS
      server: add LIMITS RCPTMAX support
      client: add Client.MaxMessageSize
      client: drop unnecessary nil check in Client.Extension
      client: drop unused Client.auth
      client: add Client.SupportsAuth
      server: send error response on invalid AUTH initial response
      server: drop Server.caps
      Add BackendFunc
      server: use Conn.authAllowed in handleAuth
      server: replace EnableAuth with AuthSession
      Disable AUTH when AuthSession.AuthMechanisms is empty
      Drop Session.AuthPlain
      Drop Server.AuthDisabled