diff --git a/cmd/logger.go b/cmd/logger.go
index 6c12e4bb3..694e944f1 100644
--- a/cmd/logger.go
+++ b/cmd/logger.go
@@ -9,6 +9,7 @@ import (
 	"path/filepath"
 	"time"
 
+	"github.com/dagu-org/dagu/internal/cmdutil"
 	"github.com/dagu-org/dagu/internal/config"
 	"github.com/dagu-org/dagu/internal/fileutil"
 	"github.com/dagu-org/dagu/internal/logger"
@@ -52,17 +53,23 @@ type logFileSettings struct {
 // openLogFile creates and opens a log file based on the provided settings.
 // It creates the necessary directory structure and returns the file handle.
 func openLogFile(config logFileSettings) (*os.File, error) {
+	logDir, err := cmdutil.SubstituteCommands(os.ExpandEnv(config.LogDir))
+	if err != nil {
+		return nil, fmt.Errorf("failed to expand log directory: %w", err)
+	}
+	config.LogDir = logDir
+
 	if err := validateSettings(config); err != nil {
 		return nil, fmt.Errorf("invalid log settings: %w", err)
 	}
 
-	logDir, err := setupLogDirectory(config)
+	outputDir, err := setupLogDirectory(config)
 	if err != nil {
 		return nil, fmt.Errorf("failed to setup log directory: %w", err)
 	}
 
 	filename := buildLogFilename(config)
-	return createLogFile(filepath.Join(logDir, filename))
+	return createLogFile(filepath.Join(outputDir, filename))
 }
 
 // validateSettings ensures all required fields are properly set
diff --git a/go.mod b/go.mod
index f1255418e..df54bbb80 100644
--- a/go.mod
+++ b/go.mod
@@ -98,6 +98,7 @@ require (
 	github.com/felixge/httpsnoop v1.0.4 // indirect
 	github.com/firefart/nonamedreturns v1.0.5 // indirect
 	github.com/fzipp/gocyclo v0.6.0 // indirect
+	github.com/gabriel-vasile/mimetype v1.4.3 // indirect
 	github.com/ghostiam/protogetter v0.3.8 // indirect
 	github.com/go-critic/go-critic v0.11.5 // indirect
 	github.com/go-logr/logr v1.4.2 // indirect
@@ -106,6 +107,9 @@ require (
 	github.com/go-openapi/inflect v0.21.0 // indirect
 	github.com/go-openapi/jsonpointer v0.21.0 // indirect
 	github.com/go-openapi/jsonreference v0.21.0 // indirect
+	github.com/go-playground/locales v0.14.1 // indirect
+	github.com/go-playground/universal-translator v0.18.1 // indirect
+	github.com/go-playground/validator/v10 v10.23.0 // indirect
 	github.com/go-toolsmith/astcast v1.1.0 // indirect
 	github.com/go-toolsmith/astcopy v1.1.0 // indirect
 	github.com/go-toolsmith/astequal v1.2.0 // indirect
@@ -158,6 +162,7 @@ require (
 	github.com/lasiar/canonicalheader v1.1.2 // indirect
 	github.com/ldez/gomoddirectives v0.2.4 // indirect
 	github.com/ldez/tagliatelle v0.5.0 // indirect
+	github.com/leodido/go-urn v1.4.0 // indirect
 	github.com/leonklingele/grouper v1.1.2 // indirect
 	github.com/macabu/inamedparam v0.1.3 // indirect
 	github.com/magiconair/properties v1.8.7 // indirect
diff --git a/go.sum b/go.sum
index bb4771b18..d06fca4af 100644
--- a/go.sum
+++ b/go.sum
@@ -191,6 +191,8 @@ github.com/fsnotify/fsnotify v1.7.0 h1:8JEhPFa5W2WU7YfeZzPNqzMP6Lwt7L2715Ggo0nos
 github.com/fsnotify/fsnotify v1.7.0/go.mod h1:40Bi/Hjc2AVfZrqy+aj+yEI+/bRxZnMJyTJwOpGvigM=
 github.com/fzipp/gocyclo v0.6.0 h1:lsblElZG7d3ALtGMx9fmxeTKZaLLpU8mET09yN4BBLo=
 github.com/fzipp/gocyclo v0.6.0/go.mod h1:rXPyn8fnlpa0R2csP/31uerbiVBugk5whMdlyaLkLoA=
+github.com/gabriel-vasile/mimetype v1.4.3 h1:in2uUcidCuFcDKtdcBxlR0rJ1+fsokWf+uqxgUFjbI0=
+github.com/gabriel-vasile/mimetype v1.4.3/go.mod h1:d8uq/6HKRL6CGdk+aubisF/M5GcPfT7nKyLpA0lbSSk=
 github.com/ghostiam/protogetter v0.3.8 h1:LYcXbYvybUyTIxN2Mj9h6rHrDZBDwZloPoKctWrFyJY=
 github.com/ghostiam/protogetter v0.3.8/go.mod h1:WZ0nw9pfzsgxuRsPOFQomgDVSWtDLJRfQJEhsGbmQMA=
 github.com/go-chi/chi/v5 v5.0.8 h1:lD+NLqFcAi1ovnVZpsnObHGW4xb4J8lNmoYVfECH1Y0=
@@ -233,6 +235,12 @@ github.com/go-openapi/swag v0.23.0 h1:vsEVJDUo2hPJ2tu0/Xc+4noaxyEffXNIs3cOULZ+Gr
 github.com/go-openapi/swag v0.23.0/go.mod h1:esZ8ITTYEsH1V2trKHjAN8Ai7xHb8RV+YSZ577vPjgQ=
 github.com/go-openapi/validate v0.24.0 h1:LdfDKwNbpB6Vn40xhTdNZAnfLECL81w+VX3BumrGD58=
 github.com/go-openapi/validate v0.24.0/go.mod h1:iyeX1sEufmv3nPbBdX3ieNviWnOZaJ1+zquzJEf2BAQ=
+github.com/go-playground/locales v0.14.1 h1:EWaQ/wswjilfKLTECiXz7Rh+3BjFhfDFKv/oXslEjJA=
+github.com/go-playground/locales v0.14.1/go.mod h1:hxrqLVvrK65+Rwrd5Fc6F2O76J/NuW9t0sjnWqG1slY=
+github.com/go-playground/universal-translator v0.18.1 h1:Bcnm0ZwsGyWbCzImXv+pAJnYK9S473LQFuzCbDbfSFY=
+github.com/go-playground/universal-translator v0.18.1/go.mod h1:xekY+UJKNuX9WP91TpwSH2VMlDf28Uj24BCp08ZFTUY=
+github.com/go-playground/validator/v10 v10.23.0 h1:/PwmTwZhS0dPkav3cdK9kV1FsAmrL8sThn8IHr/sO+o=
+github.com/go-playground/validator/v10 v10.23.0/go.mod h1:dbuPbCMFw/DrkbEynArYaCwl3amGuJotoKCe95atGMM=
 github.com/go-quicktest/qt v1.101.0 h1:O1K29Txy5P2OK0dGo59b7b0LR6wKfIhttaAhHUyn7eI=
 github.com/go-quicktest/qt v1.101.0/go.mod h1:14Bz/f7NwaXPtdYEgzsx46kqSxVwTbzVZsDC26tQJow=
 github.com/go-resty/resty/v2 v2.7.0 h1:me+K9p3uhSmXtrBZ4k9jcEAfJmuC8IivWHwaLZwPrFY=
@@ -455,6 +463,8 @@ github.com/ldez/gomoddirectives v0.2.4 h1:j3YjBIjEBbqZ0NKtBNzr8rtMHTOrLPeiwTkfUJ
 github.com/ldez/gomoddirectives v0.2.4/go.mod h1:oWu9i62VcQDYp9EQ0ONTfqLNh+mDLWWDO+SO0qSQw5g=
 github.com/ldez/tagliatelle v0.5.0 h1:epgfuYt9v0CG3fms0pEgIMNPuFf/LpPIfjk4kyqSioo=
 github.com/ldez/tagliatelle v0.5.0/go.mod h1:rj1HmWiL1MiKQuOONhd09iySTEkUuE/8+5jtPYz9xa4=
+github.com/leodido/go-urn v1.4.0 h1:WT9HwE9SGECu3lg4d/dIA+jxlljEa1/ffXKmRjqdmIQ=
+github.com/leodido/go-urn v1.4.0/go.mod h1:bvxc+MVxLKB4z00jd1z+Dvzr47oO32F/QSNjSBOlFxI=
 github.com/leonklingele/grouper v1.1.2 h1:o1ARBDLOmmasUaNDesWqWCIFH3u7hoFlM84YrjT3mIY=
 github.com/leonklingele/grouper v1.1.2/go.mod h1:6D0M/HVkhs2yRKRFZUoGjeDy7EZTfFBE9gl4kjmIGkA=
 github.com/macabu/inamedparam v0.1.3 h1:2tk/phHkMlEL/1GNe/Yf6kkR/hkcUdAEY3L0hjYV1Mk=
diff --git a/internal/agent/agent.go b/internal/agent/agent.go
index af8d5630c..2984f2ac6 100644
--- a/internal/agent/agent.go
+++ b/internal/agent/agent.go
@@ -17,6 +17,7 @@ import (
 	"time"
 
 	"github.com/dagu-org/dagu/internal/client"
+	"github.com/dagu-org/dagu/internal/cmdutil"
 	"github.com/dagu-org/dagu/internal/digraph"
 	"github.com/dagu-org/dagu/internal/digraph/scheduler"
 	"github.com/dagu-org/dagu/internal/logger"
@@ -308,19 +309,23 @@ func (a *Agent) HandleHTTP(ctx context.Context) sock.HTTPHandlerFunc {
 
 // setup the agent instance for DAG execution.
 func (a *Agent) setup(ctx context.Context) error {
-	// Lock to prevent race condition.
 	a.lock.Lock()
 	defer a.lock.Unlock()
 
 	a.scheduler = a.newScheduler()
-	a.reporter = newReporter(
-		mailer.New(&mailer.NewMailerArgs{
-			Host:     a.dag.SMTP.Host,
-			Port:     a.dag.SMTP.Port,
-			Username: a.dag.SMTP.Username,
-			Password: a.dag.SMTP.Password,
-		}),
-	)
+
+	mailerConfig, err := cmdutil.SubstituteStringFields(mailer.Config{
+		Host:     a.dag.SMTP.Host,
+		Port:     a.dag.SMTP.Port,
+		Username: a.dag.SMTP.Username,
+		Password: a.dag.SMTP.Password,
+	})
+	if err != nil {
+		return fmt.Errorf("failed to substitute mailer options: %w", err)
+	}
+
+	mailer := mailer.New(mailerConfig)
+	a.reporter = newReporter(mailer)
 
 	return a.setupGraph(ctx)
 }
diff --git a/internal/cmdutil/cmdutil.go b/internal/cmdutil/cmdutil.go
index 73a9ec22d..b705d5a40 100644
--- a/internal/cmdutil/cmdutil.go
+++ b/internal/cmdutil/cmdutil.go
@@ -236,8 +236,6 @@ func GetShellCommand(configuredShell string) string {
 // modified struct value.
 func SubstituteStringFields[T any](obj T) (T, error) {
 	v := reflect.ValueOf(obj)
-	k := v.Kind()
-	println(k)
 	if v.Kind() != reflect.Struct {
 		return obj, fmt.Errorf("input must be a struct, got %T", obj)
 	}
diff --git a/internal/digraph/builder_dag.go b/internal/digraph/builder_dag.go
index 6a64f6110..bf1d57d25 100644
--- a/internal/digraph/builder_dag.go
+++ b/internal/digraph/builder_dag.go
@@ -8,8 +8,6 @@ import (
 	"os"
 	"strings"
 	"time"
-
-	"github.com/dagu-org/dagu/internal/cmdutil"
 )
 
 var builderRegistry = []builderEntry{
@@ -109,12 +107,8 @@ func buildSteps(ctx BuildContext, spec *definition, dag *DAG) error {
 
 // buildLogDir builds the log directory for the DAG.
 func buildLogDir(_ BuildContext, spec *definition, dag *DAG) (err error) {
-	logDir, err := cmdutil.SubstituteCommands(os.ExpandEnv(spec.LogDir))
-	if err != nil {
-		return err
-	}
-	dag.LogDir = logDir
-	return err
+	dag.LogDir = spec.LogDir
+	return nil
 }
 
 // buildHandlers builds the handlers for the DAG.
diff --git a/internal/digraph/dag.go b/internal/digraph/dag.go
index 45651d4e0..6a81d6759 100644
--- a/internal/digraph/dag.go
+++ b/internal/digraph/dag.go
@@ -45,9 +45,6 @@ type DAG struct {
 	// replaced with '_').
 	LogDir string `json:"LogDir"`
 
-	// LogFormat is the format of the log (text or json).
-	LogFormat string `json:"LogFormat"`
-
 	// Parameters configuration.
 	// The DAG definition contains only DefaultParams. Params are automatically
 	// set by the DAG loader.
diff --git a/internal/digraph/executors/mail.go b/internal/digraph/executors/mail.go
index dc82d4777..c7b19f599 100644
--- a/internal/digraph/executors/mail.go
+++ b/internal/digraph/executors/mail.go
@@ -9,11 +9,15 @@ import (
 	"io"
 	"os"
 
+	"github.com/dagu-org/dagu/internal/cmdutil"
 	"github.com/dagu-org/dagu/internal/digraph"
 	"github.com/dagu-org/dagu/internal/mailer"
+	govalidator "github.com/go-playground/validator/v10"
 	"github.com/mitchellh/mapstructure"
 )
 
+var validate = govalidator.New()
+
 type mail struct {
 	stdout io.Writer
 	stderr io.Writer
@@ -22,35 +26,47 @@ type mail struct {
 }
 
 type mailConfig struct {
-	From    string `mapstructure:"from"`
-	To      string `mapstructure:"to"`
-	Subject string `mapstructure:"subject"`
+	From    string `mapstructure:"from" validate:"required,email"`
+	To      string `mapstructure:"to" validate:"required,email"`
+	Subject string `mapstructure:"subject" validate:"required"`
 	Message string `mapstructure:"message"`
 }
 
 func newMail(ctx context.Context, step digraph.Step) (Executor, error) {
-	var cfg mailConfig
-	if err := decodeMailConfig(step.ExecutorConfig.Config, &cfg); err != nil {
+	var mailConfig mailConfig
+	if err := decodeMailConfig(step.ExecutorConfig.Config, &mailConfig); err != nil {
 		return nil, err
 	}
 
-	cfg.From = os.ExpandEnv(cfg.From)
-	cfg.To = os.ExpandEnv(cfg.To)
-	cfg.Subject = os.ExpandEnv(cfg.Subject)
-	cfg.Message = os.ExpandEnv(cfg.Message)
+	mailConfig, err := cmdutil.SubstituteStringFields(mailConfig)
+	if err != nil {
+		return nil, fmt.Errorf("failed to substitute mail config: %w", err)
+	}
+
+	// Validate the configuration.
+	if err := validate.Struct(mailConfig); err != nil {
+		return nil, fmt.Errorf("invalid mail configuration: %w", err)
+	}
 
-	exec := &mail{cfg: &cfg}
+	exec := &mail{cfg: &mailConfig}
 
 	dagCtx, err := digraph.GetContext(ctx)
 	if err != nil {
 		return nil, err
 	}
-	m := mailer.New(&mailer.NewMailerArgs{
+
+	mailerConfig, err := cmdutil.SubstituteStringFields(mailer.Config{
 		Host:     dagCtx.DAG.SMTP.Host,
 		Port:     dagCtx.DAG.SMTP.Port,
 		Username: dagCtx.DAG.SMTP.Username,
 		Password: dagCtx.DAG.SMTP.Password,
 	})
+
+	if err != nil {
+		return nil, fmt.Errorf("failed to substitute mailer options: %w", err)
+	}
+
+	m := mailer.New(mailerConfig)
 	exec.mailer = m
 
 	return exec, nil
diff --git a/internal/digraph/executors/mail_test.go b/internal/digraph/executors/mail_test.go
new file mode 100644
index 000000000..5015b2149
--- /dev/null
+++ b/internal/digraph/executors/mail_test.go
@@ -0,0 +1,93 @@
+package executors
+
+import (
+	"context"
+	"os"
+	"testing"
+
+	"github.com/dagu-org/dagu/internal/digraph"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestMail(t *testing.T) {
+	t.Parallel()
+
+	os.Setenv("MAIL_SUBJECT", "Test Subject")
+	t.Cleanup(func() {
+		os.Unsetenv("MAIL_SUBJECT")
+	})
+
+	t.Run("NewMail", func(t *testing.T) {
+		tests := []struct {
+			name          string
+			step          digraph.Step
+			expectedError bool
+		}{
+			{
+				name: "ValidConfig",
+				step: digraph.Step{
+					ExecutorConfig: digraph.ExecutorConfig{
+						Config: map[string]any{
+							"from":    "test@example.com",
+							"to":      "recipient@example.com",
+							"subject": "Test Subject",
+							"message": "Test Message",
+						},
+					},
+				},
+				expectedError: false,
+			},
+			{
+				name: "ValidConfigWithEnv",
+				step: digraph.Step{
+					ExecutorConfig: digraph.ExecutorConfig{
+						Config: map[string]any{
+							"from":    "test@example.com",
+							"to":      "recipient@example.com",
+							"subject": "${MAIL_SUBJECT}",
+							"message": "Test Message",
+						},
+					},
+				},
+				expectedError: false,
+			},
+			{
+				name: "InvalidConfig",
+				step: digraph.Step{
+					ExecutorConfig: digraph.ExecutorConfig{
+						Config: map[string]any{
+							"from": "test@example.com",
+						},
+					},
+				},
+				expectedError: true,
+			},
+		}
+
+		for _, tt := range tests {
+			t.Run(tt.name, func(t *testing.T) {
+				ctx := context.Background()
+				ctx = digraph.NewContext(ctx, &digraph.DAG{
+					SMTP: &digraph.SMTPConfig{},
+				}, nil, nil, "", "")
+
+				exec, err := newMail(ctx, tt.step)
+
+				if tt.expectedError {
+					assert.Error(t, err)
+					assert.Nil(t, exec)
+				} else {
+					assert.NoError(t, err)
+					assert.NotNil(t, exec)
+
+					mailExec, ok := exec.(*mail)
+					assert.True(t, ok)
+					assert.Equal(t, "test@example.com", mailExec.cfg.From)
+					assert.Equal(t, "recipient@example.com", mailExec.cfg.To)
+					assert.Equal(t, "Test Subject", mailExec.cfg.Subject)
+					assert.Equal(t, "Test Message", mailExec.cfg.Message)
+				}
+			})
+		}
+	})
+}
diff --git a/internal/mailer/mailer.go b/internal/mailer/mailer.go
index 961798939..28d7fb17e 100644
--- a/internal/mailer/mailer.go
+++ b/internal/mailer/mailer.go
@@ -23,15 +23,15 @@ type Mailer struct {
 	password string
 }
 
-// NewMailerArgs is a config for SMTP mailer.
-type NewMailerArgs struct {
+// Config is a config for SMTP mailer.
+type Config struct {
 	Host     string
 	Port     string
 	Username string
 	Password string
 }
 
-func New(cfg *NewMailerArgs) *Mailer {
+func New(cfg Config) *Mailer {
 	return &Mailer{
 		host:     cfg.Host,
 		port:     cfg.Port,