com: fix weekmail for the case of non-existing email addresses

If an email address is set as destination for the Weekmail, the SMTP may
refuse it, and `smtplib` will throw a `SMTPRecipientsRefused` error,
containing the list of refused addresses. This commit provides an
interface for the weekmail sender to quickly unsubscribe the faulty
users, so that the next try sending the weekmail can be performed
successfully.
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