Before the upgrade, I had forwarding chains like:

address-a@example.com
→ address-b@example.com
→ external-recipient@example.net

Cloudron 10 migrated the former forwarding addresses into mailboxes with forwarding enabled, but these multi-stage chains no longer worked.

Mail sent to the first address was accepted and redirected to the second local mailbox, but never reached the external recipient. The messages remained in:

/home/yellowtent/boxdata/mail/haraka-queue

The queued messages confirmed that the first redirect occurred:

X-Sieve-Redirected-From: address-a@example.com
Delivered-To: address-a@example.com

After affected messages entered the queue, the entire mail service became unreliable. Roundcube reported:

SMTP Error (0): Connection to server failed.

Sending and receiving mail stopped working. Restarting the mail service did not resolve it because the problematic queue entries and forwarding configuration remained.

I recovered by:

1. Moving the four stranded messages out of haraka-queue.
2. Changing each affected mailbox to forward directly to its final external recipient, eliminating the intermediate local forwarding mailbox.
3. Restarting the mail service.
4. Replaying the preserved messages through SMTP.

All replayed messages received SMTP 250 responses and appeared as Sent in Cloudron’s event log. New mail is also working normally.

Possible reproduction

1. On Cloudron 9, create a forwarding address A that forwards to another local forwarding address B.
2. Configure B to forward externally to C.
3. Upgrade to Cloudron 10.0.1.
4. Send mail to A.
5. Check whether it reaches C or remains in haraka-queue.

It may also be reproducible directly on 10.0.1 by configuring forwarding-enabled mailboxes as:

A → B → external C

My suspicion is that the migration preserves each forwarding relationship without flattening the chain, while the new forwarding implementation does not safely handle mailbox-to-mailbox forwarding chains.