I captured Thunderbird’s client-side IMAP protocol log against both Dovecot 2.3.21.1 and 2.4.5 on the same server, account and Maildir.
With 2.3.21.1, CRAM-MD5 authentication succeeds and Thunderbird continues normally through NAMESPACE, COMPRESS, ID and SELECT:
```text
<CRAM-MD5 exchange removed>
OK [CAPABILITY ...] Logged in
SendData: 4 namespace
* NAMESPACE (("" "/")) NIL NIL
4 OK Namespace completed
SendData: 5 COMPRESS DEFLATE
5 OK Begin compression
SendData: 6 ID ("name" "Thunderbird" "version" "157.0.1")
* ID ("name" "Dovecot" "version" "2.3.21.1")
SendData: 7 select "INBOX"
* 7 EXISTS
7 OK [READ-WRITE] Select completed
```With 2.4.5, authentication also succeeds, but the connection closes immediately when Thunderbird sends its first post-authentication command:
```text
<CRAM-MD5 exchange removed>
OK [CAPABILITY ...] Logged in
login succeeded
SendData: 62 namespace
CreateNewLineFromSocket: clearing IMAP_CONNECTION_IS_OPEN - rv = 80470002 TellThreadToDie: close socket connection CreateNewLineFromSocket: (null) SetConnectionStatus(0x80470002) ```
The same sequence repeats on the second parallel Thunderbird connection and on subsequent retries. Thunderbird receives no IMAP BAD, NO or BYE response. It never reaches ID, SELECT or mailbox discovery with 2.4.5.
Commit 5100f0fd576d0600c3f8ad7bb056ae08235cab11 is already present in the source used for this Dovecot 2.4.5 build, so the observed failure occurs with that change included.
The Thunderbird timestamps are UTC. The first 2.4 failure occurred at 2026-10-02 15:31:01 UTC, corresponding to 08:31:01 local server time.
I can provide a sanitized longer Thunderbird trace and the corresponding Dovecot log interval if that would be useful.