diff --git a/backend/model/campaignTemplate.go b/backend/model/campaignTemplate.go
index 5c00b9b..6e684ce 100644
--- a/backend/model/campaignTemplate.go
+++ b/backend/model/campaignTemplate.go
@@ -125,6 +125,15 @@ func (c *CampaignTemplate) Validate() error {
 		)
 	}
 
+	// validate that smtp configuration and api sender are mutually exclusive
+	_, errSMTP := c.SMTPConfigurationID.Get()
+	_, errAPISender := c.APISenderID.Get()
+	if errSMTP == nil && errAPISender == nil {
+		return errs.NewValidationError(
+			errors.New("smtp configuration and api sender cannot both be set"),
+		)
+	}
+
 	return nil
 }
 
diff --git a/frontend/src/routes/campaign-template/+page.svelte b/frontend/src/routes/campaign-template/+page.svelte
index 4ab9832..367b0ba 100644
--- a/frontend/src/routes/campaign-template/+page.svelte
+++ b/frontend/src/routes/campaign-template/+page.svelte
@@ -101,6 +101,13 @@
 		modalText = getModalText('template', modalMode);
 	}
 
+	// clear smtp or api sender when switching template type
+	$: if (formValues.templateType === 'Email') {
+		formValues.apiSender = null;
+	} else if (formValues.templateType === 'External API') {
+		formValues.smtpConfiguration = null;
+	}
+
 	// hooks
 	onMount(() => {
 		const context = appStateService.getContext();