SMTP( Simple Mail Transfer Protocol )

SMTP (Simple Mail Transfer Protocol) is the standard protocol for sending emails between mail servers.

on OSI layer level 7
SMTP is only for sending
POP3/IMAP is used for reading emails
port 25(MTA -> MTA), 587 (submission)
Text based protocol

SMTP vs POP3, IMAP

On email system there are two objects, UA(User Agent) and MTA(Mail Transfer Agent). The UA refers to applications used for sending and receiving emails, such as Outlook, Thunderbird, or the default mail apps provided by OS. The MTA refers to email server. POP3, IMAP are protocols used by UA to retrieve and manage emails from the MTA. SMTP is used when UA requests the MTA to send an email and also manages communication between MTAs

General Email Transfer Process

User → SMTP Server → Recipient's SMTP Server → Mailbox → POP3/IMAP → User reads email

📜 SMTP workflow & cmd

1. Connect to SMTP server (TCP)
2. HELO / EHLO
3. MAIL FROM:<sender@example.com>
4. RCPT TO:<recipient@example.com>
5. DATA → Email content
6. QUIT

Command	Meaning
HELO / EHLO	Starts connection
MAIL FROM:	Specify sender email
RCPT TO:	Specify recipient email
DATA	Start entering email content
QUIT	Close the connection
RSET	Reset the connection (when client sent wrong message)

MX Record

MX (Mail Exchange) records are a type of DNS (Domain Name System) record that tell the internet where to deliver emails for a specific domain.

Workflow

When someone sends an email to user@example.com, the sender’s mail server needs to figure out which server handles mail for example.com.

Records route email traffic to correct mail servers
allow domains to have redundant mail servers for reliability
separate mail services from web services

It does this by:

Looking up the MX record for example.com using DNS.
The MX record responds with the hostname(s) of the mail server(s) that should receive the email.

nslookcup
> set q=mx
> domain.com
 
# The number is called the priority or preference value. Lower numbers have higher priority.
naver.com       mail exchanger = 20 mx6.mail.naver.com.
naver.com       mail exchanger = 10 mx2.naver.com.  //  lower number higher priority
naver.com       mail exchanger = 20 mx4.mail.naver.com.
naver.com       mail exchanger = 10 mx1.naver.com.
naver.com       mail exchanger = 10 mx3.naver.com.
naver.com       mail exchanger = 20 mx5.mail.naver.com.

Domain -> IP

lookup MX record gmail.com → MX → gmail-smtp-in.l.google.com

host name from MX record -> A/AAAA record

gmail-smtp-in.l.google.com → A/AAAA → IP addr

dig MX gmail.com
 
> gmail.com.  MX  5  gmail-smtp-in.l.google.com.
 
dig A gmail-smtp-in.l.google.com
 
> 74.125.203.27

Can UA directly sends mail to receiver MTA

Technically possible but limited ( Spam filtering, Authentication )

DKIM(DomainKeys Identified Mail) prevents spoofing (fake sender address)
SPF record (Sender Policy Framework)
checks sender's ip address is allowed IP

SMTP over TLS (STARTTLS)

TLS: after SSL 3.0

encrypt communication between server and client using TLS

Code

TLSClient::TLSClient(const std::string& ip, uint16_t port) {
    SSL_library_init(); // init to use OpenSSL lib
    SSL_load_error_strings();  // print error
    OpenSSL_add_all_algorithms();  // load all posible algorithm 
 
    // ready to create SSL context
    ctx = SSL_CTX_new(TLS_client_method());
    if (!ctx) throw std::runtime_error("Failed to create SSL_CTX");
 
    createSocketAndConnect(ip, port);
 
    ssl = SSL_new(ctx);
    if (!ssl) {
        SSL_CTX_free(ctx);
        close(sock);
        throw std::runtime_error("Failed to create SSL structure");
    }
 
    SSL_set_fd(ssl, sock);
 
    if (SSL_connect(ssl) <= 0) {
        SSL_free(ssl);
        SSL_CTX_free(ctx);
        close(sock);
        throw std::runtime_error("Failed SSL handshake");
    }
}

TLSClient::~TLSClient() {
    if (ssl) {
        SSL_shutdown(ssl);
        SSL_free(ssl);
    }
    if (ctx) SSL_CTX_free(ctx);
    if (sock >= 0) close(sock);
}

void TLSClient::createSocketAndConnect(const std::string& ip, uint16_t port) {
    // create TCP socket
    sock = socket(AF_INET, SOCK_STREAM, 0);
    if (sock < 0) {
        perror("socket");
        throw std::runtime_error("Failed to create socket");
    }
 
    sockaddr_in serverAddr{};
    serverAddr.sin_family = AF_INET;
    serverAddr.sin_port = htons(port);
 
    if (inet_pton(AF_INET, ip.c_str(), &serverAddr.sin_addr) <= 0) {
        close(sock);
        throw std::invalid_argument("Invalid IP address");
    }
 
    if (connect(sock, (sockaddr*)&serverAddr, sizeof(serverAddr)) < 0) {
        perror("connect");
        close(sock);
        throw std::runtime_error("Failed to connect");
    }
}

void TLSClient::sendMessage(const std::string& message) {
    size_t totalSent = 0;
    size_t len = message.size();
 
    while (totalSent < len) {
        int sent = SSL_write(ssl, message.data() + totalSent, (int)(len - totalSent));
        if (sent <= 0) {
            throw std::runtime_error("SSL_write failed");
        }
        totalSent += sent;
    }
}

std::string TLSClient::recvMessage(size_t bufSize) {
    char buffer[bufSize + 1];
    int received = SSL_read(ssl, buffer, (int)bufSize);
    if (received <= 0) {
        throw std::runtime_error("SSL_read failed or connection closed");
    }
    buffer[received] = '\0';
    return std::string(buffer);
}