[fix] Unexpected EOF Errors During mbsync Sessions

[oneingan]

Describe the bug

Node.js version: N/A (client-side issue with mbsync/isync 1.5.1; potential server-side IMAP compatibility problem).

OS version: NixOS (unstable, kernel 6.10.x).

Description: mbsync (isync 1.5.1) encounters "IMAP error: unexpected EOF" from imap.forwardemail.net:993 (TLSType IMAPS). For small syncs, it completes but errors after LOGOUT. For ~50+ emails, it aborts mid-sync, requiring multiple runs to finish.

Actual behavior

• Small changes: Sync succeeds, EOF after LOGOUT (no "* BYE" or "OK LOGOUT").
• Large changes: Partial sync (e.g., first run: Far +2 Near +6; second: Near +50), then EOF mid-process.
• Resumes on retry, but unreliable.

Example logs (two runs with ~50 emails):

❯ mbsync example
IMAP error: unexpected EOF from imap.forwardemail.net (121.127.44.70:993)
Channels: 1 Boxes: 6 Far: +2 *6 #0 -0 Near: +6 *0 #0 -0
~ took 16s
❯ mbsync example
IMAP error: unexpected EOF from imap.forwardemail.net (121.127.44.70:993)
Channels: 1 Boxes: 6 Far: +0 *0 #0 -0 Near: +50 *0 #0 -0

Verbose log (EOF after LOGOUT in completed sync):

(...)
F: >>> 22 LOGOUT
F: [ 41] Callback leave close_box
IMAP error: unexpected EOF from imap.forwardemail.net (121.127.44.70:993)
Channels: 1 Boxes: 6 Far: +0 *0 #0 -0 Near: +1 *0 #0 -0

Expected behavior

• Clean session closure with proper responses to CLOSE/LOGOUT.
• Full sync in one run, no mid-sync aborts.

Code to reproduce

Config (~/.config/isyncrc):

IMAPAccount example
CertificateFile /etc/ssl/certs/ca-certificates.crt
Host imap.forwardemail.net
PassCmd "echo 'REDACTED'"
Port 993
TLSType IMAPS
User *@example.com

IMAPStore example-remote
Account example

MaildirStore example-local
Inbox ~/.mail/example/Inbox
Path ~/.mail/example/
SubFolders Verbatim

Channel example
Create Near
Expunge Both
Far :example-remote:
Near :example-local:
Patterns *
Remove Both
SyncState *

Run mbsync -V example with ~50 new emails for mid-sync failure.

Checklist

• I have searched through GitHub issues for similar issues.
• I have completely read through the README and documentation.
• I have tested my code with the latest version of Node.js and this package and confirmed it is still not working. (N/A; tested with latest mbsync/ForwardEmail IMAP as of Sept 2025.)