Determines whether there was an error and raises the appropriate error
based on the reply code of the response
# File lib/net/smtp.rb, line 1059
def exception_class
case @status
when /\A4/ then SMTPServerBusy
when /\A50/ then SMTPSyntaxError
when /\A53/ then SMTPAuthenticationError
when /\A5/ then SMTPFatalError
else SMTPUnknownError
end
end