QCS sync stops working entirely, "Invalid messages were received from these addresses: SMTP:<email address>". No updates are received from the synchronization partner - it may also be listed as "not responding".
This may occur if the primary SMTP address of the branch service account has been changed. When an account's primary SMTP address changes, it also changes the "From:" address on emails sent by that account. QCS expects messages to come only from the address it has registered (when the synchronization partner was added), so it therefore considers these messages invalid.
To resolve this issue, change the primary SMTP address on the synchronization partner's service account back to what it was previously.
If that is not an option, then synchronization partner needs to export a new key file which then should be removed and re-added in the current location where this issue occurs.