email: fix EOF error when sending to Google SMTP relay

Created by: unknwon

There is a problem with our current email sending process:

Always send "localhost" in HELO <hostname> instead of the configured "domain" as the hostname.

Fixes https://github.com/sourcegraph/sourcegraph/issues/35943

Test plan

Manual e2e tests:

1. For regular SMTP: use email.smtp credentials prepopulated from the "dev-private".
2. For Google SMTP relay, change the site config:

   "email.smtp": {
   "authentication": "PLAIN",
     "username": "<you>@sourcegraph.com",
     "password": "<App password>",
     "host": "smtp-relay.gmail.com",
     "port": 587,
     "domain": "sourcegraph.com"
   },

3. Add a new valid email to your user settings.
4. Receive the verification email.