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It is probably a test to identify existing SMTP servers and/or acceptable users. So to say a first check before starting either some kind of attack or misuse (e.g. as relay).


Though OpenSSL has some quality issues, it would be quite optimistic to believe that the rest of the software that you expose to the Internet fares better. OpenSSL is one of the most attacked pieces of software because it is a high-value target: The same library is used in many servers of many types (HTTPS, SMTP, IMAP,...) so any vulnerability has a wide ...


Firstly, you're confusing SSL/TLS and its implementations. Your server might use OpenSSL, but that doesn't mean the clients that connect to it will. Potential vulnerabilities depend very much on the context. If you're worried about OpenSSL-specific zero-day vulnerabilities, you might be able to find a mail server that uses another stack. In addition, ...


I think you're overestimating the risk of enabling STARTTLS. Sure, there have been some incidents with OpenSSL recently, but does it mean we should all stop using HTTPS? In your situation, here is the trade-off: Using STARTTLS may open up security holes on your machines Not using STARTTLS will allow anyone snooping (on the network, on underwater ...


No matter if you use rate limiting or not. If you're running a public server, which accepts incoming connections, it is always and anywhere vulnerable for a DDoS. Rate limiting can be useful to not congest the system, it could be a possible countermeasure against DoS but does not, by far, protect fully against a DoS attack.

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