There are several email servers refusing connections of clients with public dynamic IP addresses. For example many smtp servers receiving emails only accept connections to clients having static IP adresses to avoid spam emails which are directly sent from computers having dynamic IP addresses.
When I looked for answers using google I only found information saying it is impossible to distinguish between static/dynamic addresses. So how do the email servers do it? Are there any databases providing information for specific IP ranges?
Have a look at the MX records of t-online.de (one of the largest German email providers). Try to connect to mx00.t-online.de:25 using Putty (raw mode) on a computer having a dynamic IP address. The mail server immediately closes the connection and does not even allow the client to send any command, while a connection of a server having a static IP is not refused.
Or have a look at the following extract of a 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 the email server for customers of Strato, a German hosting company, which also denies access to clients having a dynamic IP address. Email addresses have been changed. I used a recipient address which is acceptable for mailin.rzone.de, so there is no relay issue.
Also http://whatismyipaddress.com/blacklist-check states the following:
Just because the IP is listed with a particular blacklist does not mean that you are sending spam, just that particular blacklist suggests not to accept mail directly from that IP address. Most residential Cable/DSL IP addresses that are dynamically assigned will indicate that they are blacklisted, meaning you should be sending from your ISP's mail server, not a mail server running on your own internet connection.