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My customer has several security appliances that uses telnet to send email to the administrator within their domain. However, this will post a vulnerability where everyone could just telnet into their SEG to send out any email by spoofing the sender's email address.

Also, people from outside (internet) could just telnet into the SEG to send email. Is there any solution for this? Will blocking telnet connection from external help?

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    Do you really mean they use telnet? Or do you mean they connect to port 25 in plaintext? – MadHatter Aug 1 '13 at 10:08
  • They do something like this : "telnet mail.domain.com 25" – DroidMatt Aug 12 '13 at 2:33
  • How do you know that they do this? – MadHatter Aug 21 '13 at 17:54
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  1. The appliances probably aren't using telnet. They're probably using SMTP.

  2. Just because the appliances can make outbound connections to send email doesn't mean they accept inbound connections.

  3. Sending an email via telnet to a recipient that an SMTP server is authoritative for and relaying through that SMTP server to a recipient that the SMTP server is not authoritative for are two different things. For example, I can telnet to port 25 of any inbound Microsoft SMTP server and send an email to any Microsoft recipient. The Microsoft SMTP server accepts the email because it is authoritative for the domain to which I'm sending the email. On the other hand, if I try to send an email to a domain that the Microsoft SMTP server is not authoritative for it will refuse to accept the email because I'm trying to relay through it to another domain, which isn't allowed. I suspect you're confusing the two instances. Sending email via telnet TO a recipient that the SMTP server is authoritative for is perfectly normal and is how SMTP works. Sending email THROUGH an SMTP server to a recipient that the SMTP server is not authoritative is relaying and is generally prohibited.

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