SMTP protocol summary

SMTP - simple mail transfer protocol. Created in 1982 with further modernization in 2008 (ESMTP).

A protocol is used for mail transmission and can work with any transport protocol (TCP, UDP).

General mail exchange idea (SMTP)
General mail exchange idea
Ports:

  • 25 - mail transmission between servers (mail transfer agent)
  • 587 - mail transmission from client

On practice port 25 is used.

 Email format:

 Envelope (SMTP commands)

        Header (RFC 2822)

        Body (RFC 2822)

SMTP works in text format (request - response interaction).

Commands:

SMTP commands

SMTP responses: 

SMTP responses


Letter headers:

SMTP letter headers

Extended SMTP (ESMTP) commands:

  • EHLO - extended HELO
  • STARTTLS - to use encryption
  • SIZE - max allowed letter size
  • DSN - letter delivery confirmation

Security and spam:

SMTP does not contain data protection mechanisms.

  • The contents of MAIL FROM and FROM are not controlled in any way
  • Data is transmitted over the network in open form (except using STARTTLS)

Spam defence:

  • Checking the sender's domain via DNS
  • Mail servers only accept emails for local recipients
  • Verifying the sender's address with a digital signature
     

 

 


Comments

Post a Comment

Popular posts from this blog

How to design tiny url system

Image
This post is a personal cheatsheet on how to design tiny url system. Questions to ask: What’s the length of short url? How many urls will come? What duration to support? What chars in url are allowed? Basic calculation: Let’s assume that we are allowed to use chars  a-z, A-Z, 0-9 (62 chars in total). Url with length of 1 char means we are able to generate 62 unique short urls, 2 chars - \(62^2\) urls, 3 chars - \(62^3\), etc… Let’s assume that we get x requests per second and the business requirement is to store the short versions of urls for 10 years. We get following equation: \(x\) requests per second * 60 sec * 60 mins * 24 hours * 365 days * 10 years = \(y\) We get following formula from equation: \(62^n > y => n = log_{62}(y)\), where n is the length of short url. n = 7 looks like a decent amount of unique urls (\(62^7\) = 3.52 trillions) System capacity: Let’s assume that we have have read/write ratio 200:1, number of generated links per month - 100 million. Write load
Read more

System design tips on how to make capacity estimation

Image
Estimation is an important skill in system design interviews because it allows you to demonstrate your ability to reason about the complexity and feasibility of a given problem. It is worth noting that estimation is an inexact science and that it is normal for estimates to be off by some margin. It is more important to show that you have a clear understanding of the problem and can provide a thoughtful and well-reasoned estimate than to provide a perfectly accurate one. Here are a few examples of the types of estimation questions you might encounter in a system design interview: How many users can a system support? For example, you might be asked to estimate the number of users that a social media platform can support, based on the number of servers, storage, and bandwidth available. How much data can a system store or process? For example, you might be asked to estimate the amount of data that a data warehouse can store, based on the size of the data, the number of serv
Read more

IMAP protocol summary

Image
IMAP - internet message access protocol. Versions : IMAP1 (Interim Mail Access Protocol) - 1986. IMAP2 (Interactive Mail Access Protocol) - 1988. IMAP3 - 1991. IMAP4 - 1994, last updates - 2003. IMAP is used to read emails that came to the user's mailbox. Emails are stored on server :  Clients connect to the server and download emails only after a user request.  The server can perform complex operations with emails. Advantages : Several clients can work at the same time. All clients see the same mailbox status. Disadvantages :  Protocol is more complex in comparison to POP3. Server space for the mailbox is usually limited. IMAP is an application layer protocol and uses the TCP transport layer protocol. Port 143. IMAP allows you to use multiple mailboxes or folders: Folders are stored on server. Folders can form a hierarchy. Emails can be moved from one folder to another. Flag : It's an email "label". Emails can have one or multiple flags. System flags: \Seen \Answe
Read more