On successful submission of a password reset request, an email is sent
to the accounts known to the system. If sending this email fails (due to
email backend misconfiguration, service provider outage, network issues,
etc.), an attacker might exploit this by detecting which password reset
requests succeed and which ones generate a 500 error response.
Thanks to Thibaut Spriet for the report, and to Mariusz Felisiak, Adam
Johnson, and Sarah Boyce for the reviews.