Reporting wrong error when connection fails

There's a case where the error report shows:
> Couldn't connect to domain. We couldn't find any MX records for domain!

even if MXs were found, but all of the mailboxes experience a connection failure.

No file, test, or entry point is named. Start by locating the frontend code that produces the “Couldn't connect to domain” report and trace how MX discovery is distinguished from mailbox connection failures. Done means the report no longer says MX records were missing when they were found, including when every mailbox connection fails.

Written by the indexing model from the issue text.