diff --git a/src/helpers/configValidator.js b/src/helpers/configValidator.js
index 6aca537b..bf97cee1 100644
--- a/src/helpers/configValidator.js
+++ b/src/helpers/configValidator.js
@@ -5,6 +5,16 @@ const configSchema = require('./schemas/config.schema.json');
 
 const ajv = new Ajv({ logger: false, allErrors: true });
 ajv.addMetaSchema(metaSchema);
+
+ajv.addFormat('comma-separated-emails', {
+  type: 'string',
+  validate: (emails) => {
+    // this is Ajv's regex for email format (https://github.com/ajv-validator/ajv-formats/blob/master/src/formats.ts#L106)
+    const emailRegex = new RegExp(/^[a-z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-z0-9](?:[a-z0-9-]{0,61}[a-z0-9])?(?:\.[a-z0-9](?:[a-z0-9-]{0,61}[a-z0-9])?)*$/i);
+    return emails.split(',').every((email) => emailRegex.test(email.trim()));
+  },
+});
+
 const validator = ajv.addSchema(configSchema, 'config');
 
 function validateConfig(config) {
diff --git a/src/helpers/schemas/config.schema.json b/src/helpers/schemas/config.schema.json
index 8df6e9ff..fd66a709 100644
--- a/src/helpers/schemas/config.schema.json
+++ b/src/helpers/schemas/config.schema.json
@@ -49,12 +49,14 @@
         "to": {
           "anyOf": [
             {
-              "type": "string"
+              "type": "string",
+              "format": "comma-separated-emails"
             },
             {
               "type": "array",
               "items": {
-                "type": "string"
+                "type": "string",
+                "format": "email"
               }
             }
           ]