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About Port 25 blocking
Using your PCs as a mail server, you won't be able to send-out emails to recipients
whose domain reside in ISPs that have blocked their port 25
ISPs
(Internet Service Providers) are
blocking Port 25
because all mails sent via the Internet is routed through it. Because it is
the standard communication SMTP channel used for communication between a mail client (ie: Eudora, Netscape
Communicator, Outlook) and a mail server.
When an ISPs block its port 25
it does not
literally (total) block it, what it does is to check the sender IP or email
domain that is listed on their "blacklist" database and block your mail server
from sending mails to it.
For example: If you are sending a lot of email messages to the same email domain like;
hotmail.com, aol.com, or yahoo.com
their SMTP mail servers may block your email domain or IP after a certain
number of email messages--because they may have set their mail servers to
block anyone that have sent more than 100 messages within a short period of
time.
In other words, they can limit
the volume of emails from any mail server by time and/or quantity as well as permanently barred (blacklist) an email domain
or IP so that in future all all email from the IP or domain will be
automatically deleted without notifying the sender.
Please note that the PORT 25
that your program uses to send email messages to recipient mailboxes and the port
the program uses to establish connections with your email program is not the
same thing.
See:
Port Numbers
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