I am trying to create new tickets in help desk but I am facing the following error when I click on the save button. I configure the incoming and outgoing mail server on zoho mail.

Traceback (most recent call last):
  File "/home/odoo/odoo-server/odoo/http.py", line 646, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/home/odoo/odoo-server/odoo/http.py", line 307, in _handle_exception
    raise pycompat.reraise(type(exception), exception, sys.exc_info()[2])
  File "/home/odoo/odoo-server/odoo/tools/pycompat.py", line 87, in reraise
    raise value
  File "/home/odoo/odoo-server/odoo/http.py", line 683, in dispatch
    result = self._call_function(**self.params)
  File "/home/odoo/odoo-server/odoo/http.py", line 339, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/home/odoo/odoo-server/odoo/service/model.py", line 97, in wrapper
    return f(dbname, *args, **kwargs)
  File "/home/odoo/odoo-server/odoo/http.py", line 332, in checked_call
    result = self.endpoint(*a, **kw)
  File "/home/odoo/odoo-server/odoo/http.py", line 927, in __call__
    return self.method(*args, **kw)
  File "/home/odoo/odoo-server/odoo/http.py", line 512, in response_wrap
    response = f(*args, **kw)
  File "/home/odoo/enterprise/addons/web/controllers/main.py", line 924, in call_kw
    return self._call_kw(model, method, args, kwargs)
  File "/home/odoo/enterprise/addons/web/controllers/main.py", line 916, in _call_kw
    return call_kw(request.env[model], method, args, kwargs)
  File "/home/odoo/odoo-server/odoo/api.py", line 687, in call_kw
    return call_kw_model(method, model, args, kwargs)
  File "/home/odoo/odoo-server/odoo/api.py", line 672, in call_kw_model
    result = method(recs, *args, **kwargs)
  File "/home/odoo/enterprise/addons/helpdesk/models/helpdesk_ticket.py", line 243, in create
    ticket = super(HelpdeskTicket, self.with_context(mail_create_nolog=True)).create(vals)
  File "/home/odoo/enterprise/addons/mail/models/mail_thread.py", line 243, in create
    thread.message_auto_subscribe(list(create_values), values=create_values)
  File "/home/odoo/enterprise/addons/mail/models/mail_thread.py", line 2166, in message_auto_subscribe
    self._message_auto_subscribe_notify(user_pids)
  File "/home/odoo/enterprise/addons/mail/models/mail_thread.py", line 2094, in _message_auto_subscribe_notify
    subtype_id=self.env.ref('mail.mt_note').id)
  File "/home/odoo/enterprise/addons/mail/models/mail_thread.py", line 1940, in message_post_with_view
    record.message_post_with_template(False, **kwargs)
  File "/home/odoo/enterprise/addons/mail/models/mail_thread.py", line 1971, in message_post_with_template
    return composer.send_mail()
  File "/home/odoo/enterprise/addons/sale/wizard/mail_compose_message.py", line 17, in send_mail
    return super(MailComposeMessage, self).send_mail(auto_commit=auto_commit)
  File "/home/odoo/enterprise/addons/account/models/account_invoice.py", line 1818, in send_mail
    return super(MailComposeMessage, self).send_mail(auto_commit=auto_commit)
  File "/home/odoo/enterprise/addons/mail/wizard/mail_compose_message.py", line 261, in send_mail
    batch_mails.send(auto_commit=auto_commit)
  File "/home/odoo/enterprise/addons/mail/models/mail_mail.py", line 248, in send
    smtp_session.quit()
  File "/usr/lib/python3.5/smtplib.py", line 978, in quit
    res = self.docmd("quit")
  File "/usr/lib/python3.5/smtplib.py", line 419, in docmd
    self.putcmd(cmd, args)
  File "/usr/lib/python3.5/smtplib.py", line 366, in putcmd
    self.send(str)
  File "/usr/lib/python3.5/smtplib.py", line 358, in send
    raise SMTPServerDisconnected('please run connect() first')
smtplib.SMTPServerDisconnected: please run connect() first