Debian 13（trixie）を保ったまま Dovecot 2.4 → 2.3（Debian 12版）にダウングレードする手順

動作確認

Copy/usr/sbin/dovecot --version
doveconf -n | sed -n '1,120p'
ss -lntp | grep -E ':(110|143|993|995)\b'
journalctl -u dovecot -b -n 100 --no-pager

バージョンが 2.3.x、サービス状態が active (running) になっていれば成功です。

任意: FTS (Xapian) を 2.3 で有効化

1. プラグイン導入（Bookworm から）

Copysudo apt-get install -y -t bookworm dovecot-fts-xapian

2. mail_plugins に fts を追加

Copysudo sed -i 's/^mail_plugins = \(.*\)$/mail_plugins = \1 fts/' /etc/dovecot/conf.d/10-mail.conf

3. 2.3 互換の 90-fts.conf

Copysudo tee /etc/dovecot/conf.d/90-fts.conf >/dev/null <<'EOF'
plugin {
  fts = xapian
  fts_autoindex = yes
  # fts_autoindex_exclude = \Trash
  # fts_autoindex_exclude2 = \Junk
}
EOF
sudo dovecot -n
sudo systemctl restart dovecot

ロールバック（バグ解消後の2.4 へ復帰することを想定）

Copy# 設定を .debian13 に戻し、現行を .debian12 に退避して保全
sudo bash -c '
while IFS= read -r -d "" base; do
  [[ "$base" == *.ucf-dist || "$base" == *.debian12 || "$base" == *.debian13 ]] && continue
  f12="${base}.debian12"; f13="${base}.debian13"
  [[ -e "$f13" ]] || continue
  dst12="$f12"; [[ -e "$dst12" ]] && dst12="${f12}.$(date +%Y%m%d-%H%M%S)"
  mv -- "$base" "$dst12"
  mv -- "$f13" "$base"
done < <(find /etc/dovecot -type f -print0)
'

# apt を trixie 優先に戻す
sudo apt-mark unhold dovecot-core dovecot-imapd dovecot-pop3d dovecot-sieve
sudo rm -f /etc/apt/preferences.d/dovecot23.pref
sudo rm -f /etc/apt/sources.list.d/bookworm.list
sudo apt-get update
sudo apt-get install -y -t trixie dovecot-core dovecot-imapd dovecot-pop3d dovecot-sieve
sudo systemctl restart dovecot

付録: ワンファイル自動化スクリプト（任意）

A. Dovecot 2.3 導入・固定（bookworm 追加 + pinning + 安全導入）

Copy#!/usr/bin/env bash
# dovecot23_downgrade_onefile.sh
set -euo pipefail
BOOKWORM_VER="1:2.3.19.1+dfsg1-2.1+deb12u1"
PKGS=(dovecot-core dovecot-imapd dovecot-pop3d dovecot-sieve)
RED="\033[31m"; YEL="\033[33m"; GRN="\033[32m"; NC="\033[0m"
if [[ $EUID -ne 0 ]]; then echo -e "${RED}Run as root${NC}"; exit 1; fi
. /etc/os-release
[[ "${VERSION_CODENAME:-}" != trixie ]] && echo -e "${YEL}WARN: non-trixie${NC}"
[[ -d /etc/dovecot ]] && cp -a /etc/dovecot/. /root/backup-dovecot-$(date +%Y%m%d-%H%M%S)/ || true
cat >/etc/apt/sources.list.d/bookworm.list <<'EOF'
deb http://deb.debian.org/debian bookworm main contrib non-free non-free-firmware
deb http://security.debian.org/debian-security bookworm-security main contrib non-free non-free-firmware
deb http://deb.debian.org/debian bookworm-updates main contrib non-free non-free-firmware
EOF
mkdir -p /etc/apt/preferences.d
cat >/etc/apt/preferences.d/dovecot23.pref <<'EOF'
Package: dovecot*
Pin: release n=bookworm
Pin-Priority: 1001

Package: dovecot*
Pin: release n=trixie
Pin-Priority: -1
EOF
apt-get update -y
systemctl stop dovecot 2>/dev/null || true
apt-get install -y -t bookworm \
  "dovecot-core=${BOOKWORM_VER}" \
  "dovecot-imapd=${BOOKWORM_VER}" \
  "dovecot-pop3d=${BOOKWORM_VER}" \
  "dovecot-sieve=${BOOKWORM_VER}"
apt-mark hold "${PKGS[@]}" || true
/usr/sbin/dovecot --version || true
echo -e "${GRN}Done.${NC}"

B. /etc/dovecot 設定スワップ（conf.d を含む再帰・新規昇格対応）

Copy#!/usr/bin/env bash
# dovecot_swap_configs_to_debian12.sh
set -euo pipefail
TARGET_DIR="/etc/dovecot"; DRY_RUN=0; REVERT=0
usage(){ cat <<'EOS'
Usage: sudo ./dovecot_swap_configs_to_debian12.sh [--dry-run|-n] [--revert]
  forward (default): base -> .debian13, .debian12 -> base（新規昇格あり）
  --revert:          base -> .debian12, .debian13 -> base
EOS
}
ts(){ date +%Y%m%d-%H%M%S; } ; log(){ printf "%s\n" "$*"; }
is_backup_like(){ f="$1"; [[ "$f" == *~ || "$f" == *.bak || "$f" == *.orig ]]; }
move(){ src="$1"; dst="$2"; if [[ $DRY_RUN -eq 1 ]]; then log "[DRY-RUN] mv -- '$src' '$dst'"; else mv -- "$src" "$dst"; log "[OK] mv -- '$src' '$dst'"; fi }
ensure_bak(){ want="$1"; [[ -e "$want" ]] && echo "${want}.$(ts)" || echo "$want"; }
forward_swap_existing(){
  while IFS= read -r -d '' base; do
    [[ "$base" == *.ucf-dist || "$base" == *.debian12 || "$base" == *.debian13 ]] && continue
    is_backup_like "$base" && continue
    f12="${base}.debian12"; f13="${base}.debian13"
    [[ -e "$f12" ]] || continue
    dst13="$(ensure_bak "$f13")"; move "$base" "$dst13"; move "$f12" "$base"
  done < <(find "$TARGET_DIR" -type f -print0)
}
forward_promote_missing(){
  while IFS= read -r -d '' f12; do
    base="${f12%.debian12}"; [[ -e "$base" ]] && continue
    [[ "$base" == *.ucf-dist ]] && continue
    move "$f12" "$base"
  done < <(find "$TARGET_DIR" -type f -name '*.debian12' -print0)
}
revert_swap(){
  while IFS= read -r -d '' base; do
    [[ "$base" == *.ucf-dist || "$base" == *.debian12 || "$base" == *.debian13 ]] && continue
    is_backup_like "$base" && continue
    f12="${base}.debian12"; f13="${base}.debian13"; [[ -e "$f13" ]] || continue
    dst12="$(ensure_bak "$f12")"; move "$base" "$dst12"; move "$f13" "$base"
  done < <(find "$TARGET_DIR" -type f -print0)
}
while [[ $# -gt 0 ]]; do case "$1" in -n|--dry-run) DRY_RUN=1; shift;; --revert) REVERT=1; shift;; -h|--help) usage; exit 0;; *) echo "Unknown: $1"; usage; exit 1;; esac; done
[[ $EUID -ne 0 ]] && { echo "Run as root"; exit 1; }
[[ -d "$TARGET_DIR" ]] || { echo "Not found: $TARGET_DIR"; exit 1; }
log "[INFO] TARGET_DIR=$TARGET_DIR DRY_RUN=$DRY_RUN MODE=$([[ $REVERT -eq 1 ]] && echo revert || echo forward)"
if [[ $REVERT -eq 1 ]]; then revert_swap; else forward_swap_existing; forward_promote_missing; fi
log "[DONE] .ucf-dist は変更していません。"