Recommendations for SMTP DoS [closed]

Question

I have an opened SMTP server that allows me to send emails. The problem is that I can send a massive amount of emails (80,000+) to a specific mailbox and block all the other legitimate emails (that are sent to other mailboxes).

The specific mailbox is validated by size. However all the rejected emails that are sent after the size limit are stored on the same queue of the legitimates emails.

What should be done in order to solve that issue?

Answer to dr01

Right now the service is not exposed to the Internet and I wrote my own script for the Open Relay attack. Thanks for the advise!

I cant set up an SMTP auth. The service should be opened to the Internet and any user should be able to use it, unless there is a way that I'm not aware to.

The sender could be anyone, however the recipient can be only mailboxes that under my control.

In the current implementation, when an email is sent to a fulled mailbox, the email is stored on the same queue of the legitimate emails - that's the reason why I get that issue.

I was thinking on the following steps (part of you recommendations): 1. Limit the recipient mailbox size. 2. When an email is sent to a fulled mailbox, the email will be managed in a dedicated queue, which is not come across the the second queue.

This is a theoretical move. I still need to check if it's applicable.

General steps: 1. Implement an SPF mechanism. 2. Use IPS.

Answer 1

First of all, don't run an Open Relay. It will rapidly be found and exploited by scammers and your IP will be in RBLs all around the world.

Set up SMTP auth for your server, then configure it so that each account can send a maximum amount of messages per day.

Concerning the problem on the mailbox's end, there's not much that can be done to defend oneself from mailbombing. If you can put into place message size limits and blocking of senders that go over this threshold, that would be a good thing.