Routing rules can prevent mail from being sent

What does this MR do?

If Sidekiq routing rules have been incorrectly configured and the mailers queue is missing from the configuration, all outgoing mail will not be sent.

This was not immediately apparent to me this was the problem as I had configured routing rules several weeks ago and forgot about it. I then wondered today why none of my outgoing mail was not being sent. It makes sense to me to add a mention of this on the SMTP troubleshooting page and link back to the routing rules page.

Even though we have an INFO box in https://docs.gitlab.com/ee/administration/sidekiq/processing_specific_job_classes.html#routing-rules that mentions that you must add the mailers queue to your configuration - I missed that 😅

Mailer jobs cannot be routed by routing rules, and always go to the mailers queue. When using routing rules, ensure that at least one process is listening to the mailers queue. Typically this can be placed alongside the default queue.

Related issues

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