Email

lastmod 2026-05-05created 2016-06-1538 edits

Electronic mail, or email (e-mail), refers to a system for sending and receiving letters over a computer network (mainly the Internet), and to those letters themselves. Microsoft uses the term electronic mail (electronic mail).

Commonly used methods include web mail, which you can use anywhere with an Internet connection by signing up for a service such as Hanmail, Korea's first free email, or Google's Gmail; POP3, which lets you selectively download mail to your own computer; and SMTP, which sends mail simply. In the PC communication era, people often paid to create an ID in order to use email.

Filename extensions

Upon reception of email messages, email client applications save messages in operating system files in the file system. Some clients save individual messages as separate files, while others use various database formats, often proprietary, for collective storage. A historical standard of storage is the mbox format. The specific format used is often indicated by special filename extensions:

eml: Used by many email clients including Novell GroupWise, Microsoft Outlook Express, Lotus notes, Windows Mail, Mozilla Thunderbird, and Postbox. The files contain the email contents as plain text in MIME format, containing the email header and body, including attachments in one or more of several formats.

emlx: Used by Apple Mail.

msg: Used by Microsoft Office Outlook and OfficeLogic Groupware.

mbx: Used by Opera Mail, KMail, and Apple Mail based on the mbox format.

Some applications (like Apple Mail) leave attachments encoded in messages for searching while also saving separate copies of the attachments. Others separate attachments from messages and save them in a specific directory.

Email Client

Outlook
Eppie - Encrypted p2p email client based on open protocols, used without a server provider
Agentic Inbox - Self-hosted email client with a built-in AI agent

Email Server

[12 Best Self-Hosted Email Server Platforms to Use 2023]

Modoboa: Modoboa, Open Source email server; Regain your independence and protect your privacy by installing your own email server. It takes less than 10 minutes!

: Dovecot; POP3/IMAP Server

Mailu: Mailu — Mailu, Docker based mail server

Docker Mailserver: Github - docker-mailserver; Production-ready fullstack but simple mail server (SMTP, IMAP, LDAP, Antispam, Antivirus, etc.) running inside a container.

Others:

Poste.io
iRedMail
Mail-in-a-Box
hMailServer
Apache James
WildDuck Mail Server
MailSlurper
Magma Server Daemon
exim - Mail Transfer Agent
bogofilter - Spam filter
clamav - Virus filter
mox - Modern open-source mail server
Stalwart Mail Server - Open-source mail server solution

For developers

MailDev - SMTP server for testing mail during development
MailHog - Development/testing mail server (SMTP EMail Server)
Mailpit - Email testing tool for developers

Falsehoods programmers believe about email

Falsehoods programmers believe about email | GeekNews
- Code: Falsehoods programmers believe about email

In other words, don't assume these things when writing code...

Everyone has an email address
Everyone has exactly one email address
Email addresses never change
Even if an email address changes, it is under the user's control
Even if an email address changes, it is because the user specifically requested it
Even if an email address changes, the old address keeps working/existing
An email address represents only one person
Every unique string maps to a different email address
All email systems are hosted by a centralized system
When email is sent to a user at a particular domain, it is delivered to the server matching that domain
When email is sent from a user at a particular domain, it was sent from the server matching that domain
All email comes from .com, .net, .edu, or .org addresses
You can filter email based on TLD or ccTLD
Having a particular ccTLD means the recipient prefers to receive mail in that country's language
Email addresses contain only letters
Email addresses contain only letters and numbers
Email addresses contain only letters, numbers, and common punctuation marks (punctuation mark)
Email addresses contain at least one letter
Addresses like ^_^@example.com or +&#@example.com are invalid
Email is a reliable means of delivery
Email is an instant means of delivery
Email is delivered within a few minutes of being scheduled
Email is delivered within a few hours of being scheduled
Email is delivered within a few days of being scheduled
Once email is sent, it is received soon after
When email is sent, it is immediately transmitted to the destination server
If an email bounces (bounce), the address is invalid
If an email does not bounce, the address is valid
A valid address is always valid, and an invalid address is always invalid
All email is sent via SMTP over TCP/IP port 25
All email is sent via SMTP over TCP/IP
All email is sent via SMTP over IP
All email is sent via SMTP
All email servers support the vendor extensions of "the vendor everyone uses" (Google, Microsoft, etc.)
An email can have exactly one From: address
The Date: header of an email message is legitimate (legitimate)
The Received: header of an email message can never be earlier than the Date: header
All email clients support HTML attachments
All email clients support HTML message bodies
All email clients support MIME encoding
Email is secure (Secure)
Encrypted email is secure
All email is accessed through webmail
All email is accessed through webmail or IMAP
All email is accessed through webmail, IMAP, or POP3
Nobody uses email anymore.
Everyone with a .edu address is a student
Everyone with a .edu address is a student or a professor/instructor
Students and professors use their .edu address to register all of their Internet accounts
Every university provides .edu addresses
Domains are handled the same way in email and HTTP
The size of an email attachment matches the storage size of the attachment
It is safe to extract attachment filenames as is
The +suffix in an email address can be removed ([email protected] → [email protected])
Users know their own email address
Email addresses are case-sensitive, so they can be compared with == or strcmp
If you put a URL link in an email, the user will click it first

Email obfuscation

Email obfuscation: What works in 2026? | GeekNews
- [Original] Email address obfuscation: What works in 2026?
Experiments with various HTML, CSS, and JavaScript obfuscation techniques to protect email addresses from spam harvesters, comparing their block rates
In tests on samples of 426 text and 399 link addresses, most JS-based, CSS, and SVG techniques achieved a 100% block rate
Older methods such as HTML entities and URL encoding still showed high blocking effectiveness
In contrast, displaying images, substituting symbols, and reversing text direction seriously harm accessibility and usability
As of 2026, JS transformation, AES encryption, and user-interaction methods are rated the safest and most practical means of protecting email addresses

Favorite site

Wikipedia (en) Email
What are SPF and DKIM?
An in-depth analysis of email addresses | GeekNews
- Email addresses are not a simple combination of username and domain but a system that includes complex structures, rules, and security pitfalls defined in RFC standards
- The local part (before the @) has various valid forms not commonly known, such as quoted formats, comments, and Unicode, which most real-world environments do not support
- Every email has two sender addresses, the Envelope Sender and the From header, and this difference is the root cause of spoofing and phishing
- Provider-specific behaviors such as Gmail's policy of ignoring dots, plus addressing, and domain aliases directly affect security and validation
- When building systems that collect or validate email addresses, it is essential to accurately understand the gap between RFC standards and actual behavior

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