What is SMTP?
SMTP stands
for "Simple Mail Transfer Protocol". It is utilised to handle the sending
of emails. In order to support email services, a protocol pair is required,
comprising of SMTP and POP/IMAP. Together they allow the user to send outgoing
mail and retrieve incoming mail, respectively.
The SMTP server
performs three basic functions:
- It
verifies who is sending emails through the SMTP server.
- It sends
the outgoing mail
- If the
outgoing mail can't be delivered it sends the message back to the sender
Most
people will have encountered SMTP when configuring a new email address on some
third-party email clients, such as Thunderbird; as when you configure a new
email client, you will need to configure the SMTP server configuration in order
to send outgoing emails.
POP
and IMAP
POP,
or "Post Office Protocol" and IMAP, "Internet Message Access
Protocol" are both email protocols who are responsible for the transfer of
email between a client and a mail server. The main differences is in POP's more
simplistic approach of downloading the inbox from the mail server, to the
client. Where IMAP will synchronise the current inbox, with new mail on the
server, downloading anything new. This means that changes to the inbox made on
one computer, over IMAP, will persist if you then synchronise the inbox from
another computer. The POP/IMAP server is responsible for fulfiling this
process.
How
does SMTP work?
Email
delivery functions much the same as the physical mail delivery system. The user
will supply the email (a letter) and a service (the postal delivery service), and
through a series of steps- will deliver it to the recipients inbox (postbox).
The role of the SMTP server in this service, is to act as the sorting
office, the email (letter) is picked up and sent to this server, which then
directs it to the recipient.
We
can map the journey of an email from your computer to the recipient’s like
this:
1.
The mail user agent, which is either your email client or an external program.
connects to the SMTP server of your domain, e.g. smtp.google.com.
This initiates the SMTP handshake. This connection works over the SMTP port-
which is usually 25. Once these connections have been made and validated, the
SMTP session starts.
2.
The process of sending mail can now begin. The client first submits the sender,
and recipient's email address- the body of the email and any attachments, to
the server.
3.
The SMTP server then checks whether the domain name of the recipient
and the sender is the same.
4.
The SMTP server of the sender will make a connection to the
recipient's SMTP server before relaying the email. If the recipient's server
can't be accessed, or is not available- the Email gets put into an SMTP queue.
5.
Then, the recipient's SMTP server will verify the incoming email. It
does this by checking if the domain and user name have been recognised. The
server will then forward the email to the POP or IMAP server, as shown in the
diagram above.
6.
The E-Mail will then show up in the recipient's inbox.
This
is a very simplified version of the process, and there are a lot of sub-protocols,
communications and details that haven't been included. If you're looking to
learn more about this topic, this is a really friendly to read breakdown of the
finer technical details- I actually used it to write this breakdown:
https://computer.howstuffworks.com/e-mail-messaging/email3.htm
What
runs SMTP?
SMTP Server
software is readily available on Windows server platforms, with many other
variants of SMTP being available to run on Linux.
More
Information:
Here
is a resource that explain the technical implementation, and working
of, SMTP in more detail than I have covered here.
https://www.afternerd.com/blog/smtp/
Answer the questions below
What
does SMTP stand for?
How to: This can be found in the What is SMTP? section.
Answer: simple mail transfer protocol
What
does SMTP handle the sending of? (answer in plural)
How to: This can be found in the What is SMTP? section.
Answer: emails
What
is the first step in the SMTP process?
How to: This can be found in the How does SMTP work?
section. In the breakdown number 1.
Answer: SMTP handshake
What
is the default SMTP port?
How to: This can be found in the How does SMTP work?
section. In the breakdown number 1.
Answer: 25
Where
does the SMTP server send the email if the recipient's server is not
available?
How to: This can be found in the How does SMTP work?
section. In the breakdown number 4.
Answer: SMTP queue
On
what server does the Email ultimately end up on?
How to: This can be found in the How does SMTP work?
section. In the breakdown number 5.
Answer: POP/IMAP
Can
a Linux machine run an SMTP server? (Y/N)
How to: This can be found in the What runs SMTP? section.
Answer: Y
Can
a Windows machine run an SMTP server? (Y/N)
How to: This can be found in the What runs SMTP? section.
Answer: Y
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