There are several email servers that refuse client connections to public dynamic IP addresses. For example, many SMTP servers that receive emails only accept connections to clients that have static IP addresses to avoid spam emails that are sent directly from computers with dynamic IP addresses.
When I searched for answers using google, I found information only that it is impossible to distinguish between static / dynamic addresses. So how do email servers do this? Are there databases that provide information for specific ranges of IP addresses?
Check out MX t-online.de(one of Germany’s largest email providers) records . Try connecting to mx00.t-online.de:25using Putty (raw mode) on a computer with a dynamic IP address. The mail server immediately closes the connection and does not even allow the client to send any command, while the connection to the server with a static IP address is not rejected.
Or view the following excerpt from an SMTP session:
220 mailin.rzone.de [joses mi173] ESMTP RZmta 29.19 ready
EHLO Home-PC
250-mailin.rzone.de [joses mi173] greets 87.179.163.89
250-ENHANCEDSTATUSCODES
250-8BITMIME
250-PIPELINING
250-DELIVERBY
250-SIZE 104857600
250 HELP
MAIL FROM:<sender@example.com>
250 2.1.0 <sender@example.com> Sender ok
RCPT TO:<recipient@example.org>
550 5.7.1 87.179.163.89 is a dynamic IP
This is an email server for clients of the German hosting company Strato, which also prohibits access to clients with a dynamic IP address. Email addresses have been changed. I used the recipient address acceptable for mailin.rzone.de, so there is no problem with relaying.
Also http://whatismyipaddress.com/blacklist-check states the following:
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