The primary SMTP address on the QCS service mailbox was changed to become the secondary, and later removed from the list of SMTP addresses. This caused other synchronization partners to stop communicating with this forest. How to update Email address for the QCS mailbox of synchronization partners?
QCS service account's SMTP address is part of encryption key for the corresponding sync partner. If service mailbox's SMTP address has been changed, new key needs to be generated.
Please generate a new key in the QCS console - General properties tab.
IMPORTANT: QCS may have its service mailbox's SMTP addresses cached which may result in new key still reflecting an old SMTP address for the mailbox. Change needs to be triggered so that QCS re-reads new SMTP address from AD. This can be done by changing Exchange server name on the General tab under Service Mailbox area, or by clicking the Test button.