"net_io_connectionclosed" Error despite correct smtp settings

Questions
1

I’m getting this error despite having correct SMTP settings.. trying to submit an umbraco form.

Correct me if I’m wrong:

Config:

<system.net>
<mailSettings>
  <smtp from="[email protected]">
    <network host="smtp.sendgrid.net" userName="apikey" password="myapikeyhere" port="587"/>
  </smtp>
</mailSettings></system.net>

Health check in backoffice also says smtp is working as expected. But I still get the “net_io_connectionclosed” error. Any ideas?

This is a companion discussion topic for the original entry at https://our.umbraco.com/forum/108966-net_io_connectionclosed-error-despite-correct-smtp-settings