Specifications

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11.4.1 Block Outgoing Port 25
SMTP uses TCP port 25. Some ISP’s block this port at the edge of their network. This effectively prevents
customers from using any SMTP server not under control of the ISP. ISPs like this approach because if
they get a SPAM complaint they can track down the sender since each user is authenticated.
The down side of this method is that you have to use the SMTP server provided by the ISP or use a SMTP
server on a non standard port.
11.4.2 Prevent Relaying
In this case the ISP blocks SMTP access from clients outside its network. This prevents anyone not logged
into the ISP’s network from using the ISP’s server to send mail.
This prevents the user from using the same SMTP server if they use multiple ISPs.
11.4.3 Blacklist
The ISP can subscribe to a service that lists the domain names of known Spammers. If mail arrives from a
forbidden address it is refused. Lists also exist of address blocks assigned to consumer ISP’s. Mail can be
blocked if it arrives from one of these addresses on the assumption that one should not see a SMTP on
these address ranges.
11.4.4 Reverse Name Lookup
Before accepting mail the ISP can verify the mail is from a valid domain. They can also verify the
forwarding SMTP server IP address resolves to a domain name.
11.4.5 Account Verification
Verizon has a controversial policy of only accepting outgoing mail if the email from address is a Verizon
mail account, such as [e-mail].
The downside of this policy is you cannot use Verizon outgoing SMTP server to send mail from non-
Verizon mail accounts.
11.4.6 Quantity Limits
The ISP may implement rate filters, limiting how much mail can be sent over a given period of time. This
is effective at blocking Span since they need to send a huge quantity of mail.
11.4.7 POP Authenticate Before SMTP Send
To allow customer SMTP access regardless of how they connect one technique is to force the user to
retrieve mail from the POP account before allowing SMTP access. Once the user is verified the ISP
assumes the IP address is trustworthy for a short time. This allows the customer to send mail regardless of
how they connect, unless the ISP blocks access to port 25. Web hosting services commonly allow this type
of connection since their customers use other providers to access the Internet.
11.4.8 SMTP Authentication
A cleaner method of SMTP access control is to require authentication, just like the POP server. This
allows the customer to send mail independent of how they connect, unless the connecting ISP blocks access
to port 25. This is becoming the preferred method of foreign mail access.
11.4.9 My Implementation
None of the ISP’s I use block port 25 and my domain hosting service just implemented SMTP
authentication. This allowed me to configure all mail accounts on both the workstation and laptop to send
mail using my domain SMTP server. This eliminates the need to modify SMTP based on how I connect.