Developer manual: validate and rate-limit outgoing mail in email.rst #15683

developer_manual/digging_deeper/email.rst shows how to send mail, but nothing about abuse: an app can easily turn an instance into a spam relay.

• Validate recipients with IMailer::validateMailAddress().
• Rate-limit endpoints that send mail (#[UserRateLimit], #[AnonRateLimit], or ILimiter, see digging_deeper/security.rst).
• Add a note: anonymous / public users must not be able to make the instance send mail to an address they typed in, or only with a strict rate limit.