Support AUTH LOGIN for email

Created by: alexfogg

Feature request description

Support AUTH LOGIN for Sourcegraph emails

Is your feature request related to a problem? If so, please describe.

A customer is looking to adopt code monitoring, but is blocked because we do not currently support AUTH LOGIN. Their email provider, Office365, does not support PLAIN authentication.

Describe alternatives you've considered.

The customer has worked with support and tried some internal workarounds to use PLAIN or NONE, but are at an impasse.

Additional context

• Link to SG Slack support thread.
• Customer is on a managed instance, in case that is relevant.