failed attempts at 20 minute intervals to resend deferred message

[joriki]

A message that's apparently been sitting around "deferred" since October 23 (https://abstimmen.bewegung.jetzt/admin/mailer/message/43590/change/, listed under https://abstimmen.bewegung.jetzt/admin/mailer/message) is being resent every 20 minutes, resulting in loads of failure messages in the message log. The log message for the failures is:

Call to https://api.sparkpost.com/api/v1/transmissions returned 400, errors: Message generation rejected Code: 1902 Description: recipient address was suppressed due to system policy

There are at least two things wrong with this:

1. If the recipient address was suppressed due to system policy, it makes no sense to keep trying, since it'll continue to be suppressed.
2. It makes no sense to keep sending a signup code long after it's been successfully used and has expired.

[joriki]

This apparently stopped at 17:20 on January 1: https://abstimmen.bewegung.jetzt/admin/mailer/messagelog/?result__exact=3. No more failures to send the message were logged since then (but also no success), and the message is also no longer listed under https://abstimmen.bewegung.jetzt/admin/mailer/message. I'm closing the issue; we can always reopen it if the problem reoccurs.