Email Transport Overview
When you send to multiple recipients, it is very
beneficial to know how email is transported over
the Internet. Therefore, we encourage you to read
and understand the information below.
The Internet consists of millions of computers
connected together. Some of these computers are
devoted to transporting and delivering email.
They are communicating with each other with SMTP
(Simple Mail Transfer Protocol) and therefore are
called SMTP Servers.
SMTP Servers pass email messages to each other
until it reaches its destination. When an SMTP
server receives a message, it can do one of the
1. Store the message for future retrieval by
2. Pass it to another SMTP server, which will
be able to deliver the message. This is called
3. Reject the message as undeliverable. If that
happens, the SMTP server that was trying to send
the message must notify the sender. It usually
does this by sending a special email message
There exist a special service called DNS
(Domain Name Service) which stores so called MX
records for each domain. MX record says which
SMTP server needs to be used for a particular email
So, a typical lifecycle of an email message
is as follows:
1. The sender composes a mail message in his
favorite mail client such as Outlook Express.
2. Mail client connects to the nearest SMTP
server and passes the message to it.
3. The SMTP server looks up MX records and
relays the message to another SMTP server which
is suitable for the destination email address.
4. Eventually, the message reaches its
destination SMTP server and is stored there.
5. The recipient retrieves his mail from the
destination SMTP server. Usually this is done
with POP3 (Post Office Protocol v3), and therefore
the SMTP server which is the last in the chain is often referred
to as an incoming (POP3) mail server by the recipient.
Unfortunately, mail system is severely abused by
unresponsible commercial advertisers (usually called
spammers) who try to send their messages to every
email address in the world. Many Internet Service
Providers aggressively fight spammers, and since
they do not really can tell legitimate bulk
mailers from spammers, your mass mailing may be
blocked at any stage.
Here is what you should expect:
First of all, you may not be able to even connect
to the SMTP server of your choice. For example,
Earthlink will not let their users connect to any
of SMTP servers except for their own.
Worse yet, all your connections may be silently
redirected to the SMTP server of your ISP. In this
case, all your mail will go through their server and
may be controlled, even though you may think you use
other SMTP server or send direct.
Then, the very first SMTP server you connect
to may reject you based on any of the following
1. Your Internet connection point is not recognized.
For example, if you have two Internet accounts on
Earthlink and AOL, then you connect to the Internet
using AOL but then connect to the SMTP server
belonging to Earthlink. This SMTP server recognizes
you as AOL user, not as Earthlink user, and thus
rejects all your activity.
2. Your From address does
not belong to the ISP domain. For example, you want to
send email through your ISP mail server, but spell your
return address as firstname.lastname@example.org.
3. You may be required to supply a password.
4. You may be marked as sending message to too many
recipients. Usually this limit applies to the number
of recipients per single message, which is not a real
problem. Some more sophisticated providers may limit
the number of messages that you can send per hour,
per day, or per month.
5. Some ISP may scan the contents of your message
and reject it if they find suspicious words, even
though such a behavior is illegal.
Many SMTP servers will do an excessive lookup on
all the addresses, which may cause severe delays.
Some servers delay communications intentionally to
decrease the load.
Once your message comes through the original SMTP,
it does not mean that it is going to be delivered,
as it needs to come through all filters on the receiving
1. Your email may be rejected because it is not
2. The validity of your return address may be
3. The validity of the original SMTP server may be
verified. Some of the receiving servers go very long
distance doing this verification. They may connect
to the originating SMTP server to find out if the
server would relay mail for unauthorized users. If it
does, the mail may be blocked.
4. The original SMTP server may be checked against
numerous black lists. These black lists normally include
addresses of recent spammers and open relays.
However, some of the black lists is much more
aggressive. For example, DNSBL contains a huge
list of IP addresses. They classify all the IP
addresses into Internet Service Providers and
end users. They look at the end users as potential
spammers, and block all the mail from their IPs.
Therefore, when you send directly from your computer,
part of your email may be blocked.
5. Some recipients may decide not to accept HTML messages,
because most of the unsolicited mail is in HTML format.
6. Your message may be checked against the list of
Given all these filters, it is getting increasingly
difficult to send mass email, and the problem arise now
and then with some particular recipients or with whole
lists. AY Mail will help you avoid as much of these
problems as it can, but you do have to be aware of
these numerous problems that may arise.
Direct sending versus SMTP