what is smart folder and how to get rid of error "smpt smarttls command failed"

Mail
1

get this error with every refresh

2

Are you sure it isn’t STARTTLS , which is a connection protocol security policy. Go to Menu > Tools > Accounts and make sure you are using the following combinations of port and security policy:

SMTP
Port 587 = Force usage of SSL/TLS

IMAP
Port 993 = Use SSL/TLS on special port (legacy)

or if you are using POP3:
POP3
Port 995 = Use SSL/TLS on special port (legacy)

A smart folder is a filter that you can apply to messages and folders. e.g. the default Unread Smart Folder will display messages from all accounts and folders that are unread.