Previously, there were two available options for syncing the items in your ECS mailbox sent folder back your primary mailbox. One of those options has been removed.
Option 1 - This feature has been removed
Bcc yourself on all outgoing messages from the ECS mailbox during an outage. When your mail server comes back online, the messages sent from the ECS mailbox will appear in the inbox of your primary mail account.
Option 2 - Currently the only way to retain the sent messages
Once your server is back up after an outage, add your ECS mailbox via POP/IMAP to Outlook (or other mail client) and "drag and drop" the sent items from your ECS mailbox to the sent items in your primary mailbox. The messages will then sync to your primary mail server.
Why was this feature removed?
We have a technical mail flow limitation that prevents outbound ECS messages addressed to the sender from reaching your external mail server.
Are there plans to fix or replace this functionality?
We are working on a more seamless approach to syncing the sent items.
What happens if you continue to Bcc yourself?
The messages you send from your ECS mailbox will arrive in your ECS mailbox, but will not appear in your primary mailbox once your external mail server is back online.
Has anything changed with inbound messages being received after an outage?
No, all queued inbound messages will continue to appear in your primary mailbox once your external mail server is back online.