@Deepika, my suggestion was to mute it once and check if error persists. My guess is you have incorrect email account related entries in you site_config.json
Following correct value should be present in site_config
mail_server: SMTP server hostname.
mail_port: STMP port.
use_ssl: Connect via SSL / TLS.
mail_login: Login id for SMTP server.
mail_password: Password for SMTP server.
Alternatively, you can delete email domain/account in desk and set it up again. This way you need not change or correct site_config.json values. It would be overwritten.