diff --git a/internal/imap/backend.go b/internal/imap/backend.go
index 1cfcc4b6..697f4fa7 100644
--- a/internal/imap/backend.go
+++ b/internal/imap/backend.go
@@ -32,6 +32,7 @@ import (
 )
 
 const (
+	// NOTE: Each fetch worker has its own set of attach workers so there can up to 20*5=100 API requests at once.
 	fetchWorkers  = 20 // In how many workers to fetch message (group list on IMAP).
 	attachWorkers = 5  // In how many workers to fetch attachments (for one message).
 	buildWorkers  = 20 // In how many workers to build messages.
diff --git a/pkg/message/build.go b/pkg/message/build.go
index c141cd00..a3c7e78f 100644
--- a/pkg/message/build.go
+++ b/pkg/message/build.go
@@ -45,6 +45,17 @@ type Fetcher interface {
 	KeyRingForAddressID(string) (*crypto.KeyRing, error)
 }
 
+// NewBuilder creates a new builder which manages the given number of fetch/attach/build workers.
+//  - fetchWorkers: the number of workers which fetch messages from API
+//  - attachWorkers: the number of workers which fetch attachments from API.
+//  - buildWorkers: the number of workers which decrypt/build RFC822 message literals.
+//
+// NOTE: Each fetch worker spawns a unique set of attachment workers!
+// There can therefore be up to fetchWorkers*attachWorkers simultaneous API connections.
+//
+// The returned builder is ready to handle jobs -- see (*Builder).NewJob for more information.
+//
+// Call (*Builder).Done to shut down the builder and stop all workers.
 func NewBuilder(fetchWorkers, attachWorkers, buildWorkers int) *Builder {
 	b := newBuilder()
 
@@ -98,10 +109,13 @@ func newBuilder() *Builder {
 	}
 }
 
+// NewJob tells the builder to begin building the message with the given ID.
+// The result (or any error which occurred during building) can be retrieved from the returned job when available.
 func (b *Builder) NewJob(ctx context.Context, api Fetcher, messageID string) *BuildJob {
 	return b.NewJobWithOptions(ctx, api, messageID, JobOptions{})
 }
 
+// NewJobWithOptions creates a new job with custom options. See NewJob for more information.
 func (b *Builder) NewJobWithOptions(ctx context.Context, api Fetcher, messageID string, opts JobOptions) *BuildJob {
 	b.locker.Lock()
 	defer b.locker.Unlock()
@@ -117,6 +131,7 @@ func (b *Builder) NewJobWithOptions(ctx context.Context, api Fetcher, messageID
 	return b.jobs[messageID]
 }
 
+// Done shuts down the builder and stops all workers.
 func (b *Builder) Done() {
 	b.locker.Lock()
 	defer b.locker.Unlock()
diff --git a/pkg/message/build_build.go b/pkg/message/build_build.go
index 9639c7cb..c8d386c0 100644
--- a/pkg/message/build_build.go
+++ b/pkg/message/build_build.go
@@ -43,6 +43,11 @@ func newBuildResFailure(messageID string, err error) buildRes {
 	}
 }
 
+// startBuildWorkers starts the given number of build workers.
+// These workers decrypt and build messages into RFC822 literals.
+// Two channels are returned:
+//  - buildReqCh: used to send work items to the worker pool
+//  - buildResCh: used to receive work results from the worker pool
 func startBuildWorkers(buildWorkers int) (chan fetchRes, chan buildRes) {
 	buildReqCh := make(chan fetchRes)
 	buildResCh := make(chan buildRes)
diff --git a/pkg/message/build_fetch.go b/pkg/message/build_fetch.go
index 7807be92..69e3e3da 100644
--- a/pkg/message/build_fetch.go
+++ b/pkg/message/build_fetch.go
@@ -56,6 +56,12 @@ func newFetchResFailure(req fetchReq, err error) fetchRes {
 	}
 }
 
+// startFetchWorkers starts the given number of fetch workers.
+// These workers download message and attachment data from API.
+// Each fetch worker will use up to the given number of attachment workers to download attachments.
+// Two channels are returned:
+//  - fetchReqCh: used to send work items to the worker pool
+//  - fetchResCh: used to receive work results from the worker pool
 func startFetchWorkers(fetchWorkers, attachWorkers int) (chan fetchReq, chan fetchRes) {
 	fetchReqCh := make(chan fetchReq)
 	fetchResCh := make(chan fetchRes)
diff --git a/pkg/message/build_job.go b/pkg/message/build_job.go
index 15cd74f2..b800afa7 100644
--- a/pkg/message/build_job.go
+++ b/pkg/message/build_job.go
@@ -41,6 +41,8 @@ func newBuildJob(messageID string) *BuildJob {
 	}
 }
 
+// GetResult returns the build result or any error which occurred during building.
+// If the result is not ready yet, it blocks.
 func (job *BuildJob) GetResult() ([]byte, error) {
 	<-job.done
 	return job.literal, job.err
diff --git a/pkg/message/build_rfc822_custom.go b/pkg/message/build_rfc822_custom.go
index bcedf5e8..477f0e27 100644
--- a/pkg/message/build_rfc822_custom.go
+++ b/pkg/message/build_rfc822_custom.go
@@ -27,6 +27,7 @@ import (
 	"github.com/emersion/go-message"
 )
 
+// writeCustomTextPart writes an armored-PGP text part for a message body that couldn't be decrypted.
 func writeCustomTextPart(
 	w *message.Writer,
 	msg *pmapi.Message,
@@ -61,6 +62,7 @@ func writeCustomTextPart(
 	return nil
 }
 
+// writeCustomTextPart writes an armored-PGP data part for an attachment that couldn't be decrypted.
 func writeCustomAttachmentPart(
 	w *message.Writer,
 	att *pmapi.Attachment,