Add validation for beforeSend hook to ensure required properties remain intact

- Validate that 'from' field is not removed
- Validate that 'to' field is not removed or emptied
- Validate that 'subject' field is not removed
- Validate that at least 'html' or 'text' content exists
- Throw clear error messages if validation fails
- Bump version to 0.1.21

src/services/MailingService.ts

@@ -285,6 +285,20 @@ export class MailingService implements IMailingService {
       if (this.config.beforeSend) {
         try {
           mailOptions = await this.config.beforeSend(mailOptions, email)
+
+          // Validate required properties remain intact after hook execution
+          if (!mailOptions.from) {
+            throw new Error('beforeSend hook must not remove the "from" property')
+          }
+          if (!mailOptions.to || (Array.isArray(mailOptions.to) && mailOptions.to.length === 0)) {
+            throw new Error('beforeSend hook must not remove or empty the "to" property')
+          }
+          if (!mailOptions.subject) {
+            throw new Error('beforeSend hook must not remove the "subject" property')
+          }
+          if (!mailOptions.html && !mailOptions.text) {
+            throw new Error('beforeSend hook must not remove both "html" and "text" properties')
+          }
         } catch (error) {
           console.error('Error in beforeSend hook:', error)
           throw new Error(`beforeSend hook failed: ${error instanceof Error ? error.message : 'Unknown error'}`)