Carbonio 26 – Apple Mail iPhone IMAP connects but shows empty mailbox after upgrade from 25 to 26

Hello,

We identified a potential regression in Carbonio CE 26 affecting Apple Mail on iPhone after upgrading from Carbonio CE 25.

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ENVIRONMENT
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- Product: Carbonio CE
- Upgrade: 25 → 26
- Installation: Single server
- Hostname: mail.example.net
- Domain: example.net

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CLIENTS
========================
- Affected: Apple Mail (iPhone iOS)
- NOT affected:
- Other Carbonio servers NOT upgraded (same configuration)
- Gmail IMAP
- Webmail

Important:
The same iPhone works perfectly with other Carbonio servers not upgraded to version 26.

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SYMPTOMS
========================
- IMAP authentication: OK
- SMTP sending: OK
- Webmail: OK
- Apple Mail connects successfully BUT:
→ mailbox is empty
→ no folders displayed
→ no messages

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IMAP MANUAL TEST (OK)
========================
Using openssl:

* OK IMAP4rev1 proxy server ready
a login user@example.net ****
a OK LOGIN completed

a list "" "*"
* LIST (\HasNoChildren) "/" "INBOX"
* LIST (\HasNoChildren) "/" "Sent"
* LIST (\HasNoChildren) "/" "Trash"

a select INBOX
* XXX EXISTS
* OK [UNSEEN XX]
a OK [READ-WRITE] SELECT completed

→ Messages are present and accessible via IMAP

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OBSERVED BEHAVIOR (APPLE MAIL)
========================
From mailbox.log:

Apple Mail repeatedly sends:
- NAMESPACE
- LIST

BUT never sends:
- SELECT INBOX

Example log extract:

2026-03-20 10:12:01,123 INFO [ImapSSLServer-10] imap - cmd=LOGIN
2026-03-20 10:12:01,456 INFO [ImapSSLServer-10] imap - cmd=NAMESPACE
2026-03-20 10:12:01,789 INFO [ImapSSLServer-10] imap - cmd=LIST "" "*"
2026-03-20 10:12:02,012 INFO [ImapSSLServer-10] imap - cmd=NAMESPACE
2026-03-20 10:12:02,345 INFO [ImapSSLServer-10] imap - cmd=LIST "" "*"

→ loop without SELECT INBOX

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TROUBLESHOOTING DONE
========================
- Mailbox reIndex completed successfully

- Account deleted and recreated → same issue

- Direct IMAP backend (port 7993) tested → same issue

- Standard proxy IMAPS (port 993) restored → same issue

- TLS certificate valid (Let's Encrypt)

- Services status:
- mailbox: running
- proxy: running
- mta: running

- Network (firewall / NAT) ruled out

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CONCLUSION
========================
This appears to be an IMAP compatibility issue or regression introduced in Carbonio CE 26 affecting Apple Mail.

The fact that:
- IMAP manual commands work
- Other Carbonio servers (not upgraded) work
- Only Apple Mail is impacted

strongly suggests a protocol or capability-related issue

Conclusion :
Probable bug de compatibilité IMAP introduit dans Carbonio CE 26.

Best regards

Pierre