detail cause of error for: "Content sign failed: invalid private key."

For me the case was: I had reached my quota limit
It might also have other causes.

Perhaps such a msg is fine:
"You might have sent too many mails in x h" or so

No source file, test, or entry point is named in the issue. Trace where the "Content sign failed: invalid private key" message is produced, identify how quota-limit failures are distinguished, and consider the work complete when the error explains the applicable cause instead of only reporting an invalid key.

Vom Indexierungsmodell aus dem Issue-Text verfasst.