5 files changed, 30 insertions, 3 deletions
diff --git a/internal/feed/cache.go b/internal/feed/cache.go
index f1dfb77..731eab1 100644
--- a/internal/feed/cache.go
+++ b/internal/feed/cache.go
@@ -27,6 +27,7 @@ type CachedFeed interface {
 	Checked(withFailure bool)
 	Failures() int
 	Last() time.Time
+	ID() string
 	filterItems(items []feeditem, ignoreHash bool, alwaysNew bool) []feeditem
 	Commit()
 }
diff --git a/internal/feed/cache_v1.go b/internal/feed/cache_v1.go
index bbb8f13..b2813ce 100644
--- a/internal/feed/cache_v1.go
+++ b/internal/feed/cache_v1.go
@@ -3,8 +3,11 @@ package feed
 import (
 	"crypto/sha256"
 	"fmt"
+	"strconv"
 	"time"
 
+	"github.com/lithammer/shortuuid"
+
 	"github.com/Necoro/feed2imap-go/pkg/log"
 	"github.com/Necoro/feed2imap-go/pkg/util"
 )
@@ -16,6 +19,10 @@ const (
 
 type feedId uint64
 
+func (id feedId) String() string {
+	return strconv.FormatUint(uint64(id), 16)
+}
+
 type v1Cache struct {
 	Ids    map[feedDescriptor]feedId
 	NextId uint64
@@ -23,6 +30,7 @@ type v1Cache struct {
 }
 
 type cachedFeed struct {
+	id           feedId // not saved, has to be set on loading
 	LastCheck    time.Time
 	currentCheck time.Time
 	NumFailures  int // can't be named `Failures` b/c it'll collide with the interface
@@ -40,6 +48,7 @@ type cachedItem struct {
 	UpdatedDate   time.Time
 	UpdatedCache  time.Time
 	Hash          itemHash
+	ID            string
 }
 
 func (item cachedItem) String() string {
@@ -75,6 +84,10 @@ func (cf *cachedFeed) Last() time.Time {
 	return cf.LastCheck
 }
 
+func (cf *cachedFeed) ID() string {
+	return cf.id.String()
+}
+
 func (cache *v1Cache) Version() Version {
 	return v1Version
 }
@@ -98,6 +111,7 @@ func (cache *v1Cache) getItem(id feedId) CachedFeed {
 		feed = &cachedFeed{}
 		cache.Feeds[id] = feed
 	}
+	feed.id = id
 	return feed
 }
 
@@ -142,6 +156,8 @@ func (cache *v1Cache) findItem(feed *Feed) CachedFeed {
 func newCachedItem(item feeditem) cachedItem {
 	var ci cachedItem
 
+	ci.ID = shortuuid.New()
+
 	ci.Title = item.Item.Title
 	ci.Link = item.Item.Link
 	if item.Item.PublishedParsed != nil {
@@ -192,6 +208,7 @@ func (cf *cachedFeed) filterItems(items []feeditem, ignoreHash, alwaysNew bool)
 		}
 		filtered = append(filtered, *item)
 		cacheadd = append(cacheadd, ci)
+		item.itemId = ci.ID
 	}
 
 CACHE_ITEMS:
@@ -211,6 +228,7 @@ CACHE_ITEMS:
 					log.Debugf("Guid matches with: %s", oldItem)
 					if !oldItem.similarTo(&ci, ignoreHash) {
 						item.addReason("guid (upd)")
+						ci.ID = oldItem.ID
 						app(item, ci, &idx)
 					} else {
 						log.Debugf("Similar, ignoring")
@@ -240,6 +258,7 @@ CACHE_ITEMS:
 				}
 				log.Debugf("Link matches, updating: %s", oldItem)
 				item.addReason("link (upd)")
+				ci.ID = oldItem.ID
 				app(item, ci, &idx)
 
 				continue CACHE_ITEMS
diff --git a/internal/feed/feed.go b/internal/feed/feed.go
index 1a6ef97..9ed44df 100644
--- a/internal/feed/feed.go
+++ b/internal/feed/feed.go
@@ -35,6 +35,7 @@ type feeditem struct {
 	updateOnly bool
 	reasons    []string
 	images     []feedImage
+	itemId     string
 }
 
 // Creator returns the name of the creating author.
diff --git a/internal/feed/mail.go b/internal/feed/mail.go
index 4b0e38b..ebf032a 100644
--- a/internal/feed/mail.go
+++ b/internal/feed/mail.go
@@ -49,8 +49,9 @@ func buildHeader(feed *Feed, item feeditem, cfg *config.Config) message.Header {
 	h.SetContentType("multipart/alternative", nil)
 	h.SetAddressList("From", fromAdress(feed, item, cfg))
 	h.SetAddressList("To", address(feed.Name, cfg.DefaultEmail))
-	h.Add("X-Feed2Imap-Version", config.Version())
-	h.Add("X-Feed2Imap-Reason", strings.Join(item.reasons, ","))
+	h.Set("X-Feed2Imap-Version", config.Version())
+	h.Set("X-Feed2Imap-Reason", strings.Join(item.reasons, ","))
+	h.Set("Message-Id", feed.messageId(item))
 
 	{ // date
 		date := item.Item.PublishedParsed
@@ -227,6 +228,10 @@ func getBody(content, description string, bodyCfg config.Body) string {
 	}
 }
 
+func (feed *Feed) messageId(item feeditem) string {
+	return fmt.Sprintf("<feed#%s#%s@%s>", feed.cached.ID(), item.itemId, config.Hostname())
+}
+
 func (feed *Feed) buildBody(item *feeditem) {
 	body := getBody(item.Item.Content, item.Item.Description, feed.Body)
 
diff --git a/internal/feed/parse.go b/internal/feed/parse.go
index 1ce6cda..435c0ed 100644
--- a/internal/feed/parse.go
+++ b/internal/feed/parse.go
@@ -7,6 +7,7 @@ import (
 	"net/http"
 	"time"
 
+	"github.com/lithammer/shortuuid"
 	"github.com/mmcdole/gofeed"
 
 	"github.com/Necoro/feed2imap-go/pkg/log"
@@ -55,7 +56,7 @@ func parseFeed(feed *Feed) error {
 	feed.feed = parsedFeed
 	feed.items = make([]feeditem, len(parsedFeed.Items))
 	for idx, item := range parsedFeed.Items {
-		feed.items[idx] = feeditem{Feed: parsedFeed, Item: item}
+		feed.items[idx] = feeditem{Feed: parsedFeed, Item: item, itemId: shortuuid.New()}
 	}
 	return nil
 }