feat(validation): add request and domain validation for email use case

domain/src/main/java/com/pablotj/restemailbridge/domain/service/EmailValidatorService.java

@@ -0,0 +1,22 @@
+package com.pablotj.restemailbridge.domain.service;
+
+import com.pablotj.restemailbridge.domain.model.Email;
+
+
+public class EmailValidatorService {
+
+    /**
+     * Validates business rules for Email.
+     */
+    public void validate(Email email) {
+        if (email == null) throw new IllegalArgumentException("Email cannot be null");
+        if (email.getTo() == null || !email.getTo().matches(".+@.+\\..+"))
+            throw new IllegalArgumentException("Recipient email is invalid");
+        if (email.getFrom() == null || email.getFrom().isBlank())
+            throw new IllegalArgumentException("Sender email is required");
+        if (email.getSubject() == null || email.getSubject().isBlank())
+            throw new IllegalArgumentException("Subject is required");
+        if (email.getBody() == null || email.getBody().isBlank())
+            throw new IllegalArgumentException("Body is required");
+    }
+}