diff --git a/main.go b/main.go
index c395794..0ddb676 100644
--- a/main.go
+++ b/main.go
@@ -76,6 +76,8 @@ var rootCmd = &cobra.Command{
 	RunE: func(cmd *cobra.Command, args []string) error {
 		var deliveryMethod DeliveryMethod
 		switch {
+		case resendAPIKey != "" && smtpUsername != "" && smtpPassword != "":
+			deliveryMethod = Unknown
 		case resendAPIKey != "":
 			deliveryMethod = Resend
 		case smtpUsername != "" && smtpPassword != "":
@@ -83,12 +85,20 @@ var rootCmd = &cobra.Command{
 			from = smtpUsername
 		}
 
-		if deliveryMethod == None {
+		switch deliveryMethod {
+		case None:
 			fmt.Printf("\n  %s %s %s\n\n", errorHeaderStyle.String(), inlineCodeStyle.Render(ResendAPIKey), "environment variable is required.")
 			fmt.Printf("  %s %s\n\n", commentStyle.Render("You can grab one at"), linkStyle.Render("https://resend.com/api-keys"))
 			cmd.SilenceUsage = true
 			cmd.SilenceErrors = true
 			return errors.New("missing required environment variable")
+		case Unknown:
+			fmt.Printf("\n  %s Unknown delivery method.\n", errorHeaderStyle.String())
+			fmt.Printf("\n  You have set both %s and %s delivery methods.", inlineCodeStyle.Render(ResendAPIKey), inlineCodeStyle.Render("POP_SMPT_*"))
+			fmt.Printf("\n  Set only one of these environment variables.\n\n")
+			cmd.SilenceUsage = true
+			cmd.SilenceErrors = true
+			return errors.New("unknown delivery method")
 		}
 
 		if hasStdin() {
diff --git a/model.go b/model.go
index 6488fbe..a6cf30a 100644
--- a/model.go
+++ b/model.go
@@ -41,6 +41,9 @@ const (
 	Resend
 	// SMTP uses an SMTP server to send an email.
 	SMTP
+	// Unknown is set when the user has not chosen a single delivery method.
+	// i.e. multiple delivery methods are set.
+	Unknown
 )
 
 // Model is Pop's application model.