如何通过代码实现SMTP服务器直接向其他服务器发送邮件

Got it, I totally get your frustration—every search result keeps pointing you to client-side SMTP setups, but you want the server-to-server MTA logic instead. Let's dive into what you need.

核心思路：MTA-to-MTA的SMTP交互 #

When an SMTP server sends mail to another server, it follows the standard MTA workflow:

• Step 1: Resolve the target domain's MX records — Use DNS to find which SMTP servers are responsible for accepting mail for the recipient's email domain.
• Step 2: Establish a TCP connection — Connect to the highest-priority MX server (lower priority number = higher priority) on port 25 (the standard MTA port; some use 587 for submission, but 25 is for server-to-server).
• Step 3: Perform SMTP handshake — Send EHLO (or HELO) to identify your server, then negotiate TLS if supported.
• Step 4: Send envelope details — Use MAIL FROM and RCPT TO to specify the sender and recipient.
• Step 5: Transmit the email content — Send the DATA command, followed by the full email headers and body, ending with a single . on a line.
• Step 6: Close the session — Send QUIT to terminate the connection.

常用的SMTP服务器/MTA库 #

Depending on your language, these libraries handle most low-level SMTP protocol work for you:

• Python:
  • smtplib (built-in) can be repurposed for MTA-to-MTA sending (as shown below)
  • aiosmtpd — A full-featured async SMTP server framework, ideal if you're building your own MTA that needs to receive and forward mail
• Node.js:
  • nodemailer — Configure it to skip specifying a fixed SMTP host, and it'll automatically resolve MX records for the recipient domain
  • smtp-server — Build your own MTA with full control over incoming/outgoing mail flow
• Go:
  • net/smtp (standard library) combined with net/dns to resolve MX records
  • github.com/emersion/go-smtp — A robust, extensible SMTP server implementation

Python代码示例：直接发送到目标MX服务器 #

Here's a working snippet that skips the "client connects to own SMTP server" step and goes straight to the recipient's MX server:

import smtplib
import dns.resolver
from email.mime.text import MIMEText

def send_mta_to_mta(sender: str, recipient: str, subject: str, body: str):
    # Resolve recipient domain's MX records
    try:
        recipient_domain = recipient.split("@")[1]
        mx_records = dns.resolver.resolve(recipient_domain, "MX")
    except (IndexError, dns.resolver.NXDOMAIN, dns.resolver.NoAnswer) as e:
        print(f"Failed to resolve MX records: {str(e)}")
        return

    # Pick the highest-priority MX server
    mx_server = sorted(mx_records, key=lambda r: r.preference)[0].exchange.to_text()

    # Build the email message
    msg = MIMEText(body)
    msg["From"] = sender
    msg["To"] = recipient
    msg["Subject"] = subject

    # Connect and send
    try:
        with smtplib.SMTP(mx_server, 25, timeout=30) as server:
            # EHLO to initiate session
            server.ehlo()
            # Enable TLS if supported (most modern MTAs require this)
            if server.has_extn("STARTTLS"):
                server.starttls()
                server.ehlo()  # Re-EHLO after TLS upgrade
            # Send the email
            server.sendmail(sender, recipient, msg.as_string())
        print("Email sent successfully via target MX server!")
    except Exception as e:
        print(f"Failed to send email: {str(e)}")

# Example usage
send_mta_to_mta(
    sender="your-server@your-domain.com",
    recipient="user@target-domain.com",
    subject="Test MTA-to-MTA Email",
    body="This email was sent directly to the recipient's MX server, no intermediate SMTP client required!"
)

Critical Notes for Production #

• Port 25 Restrictions: Most cloud providers (AWS, GCP, Azure) block port 25 by default to prevent spam. You'll need to request a lift on this restriction or use port 587 for submission (though some MTAs only accept server-to-server traffic on 25).
• Reverse DNS (PTR): Your server's public IP must have a valid PTR record pointing to your domain. Without this, most MTAs will mark your mail as spam or reject it outright.
• SPF/DKIM/DMARC: Set up these DNS records for your sending domain to improve deliverability and prevent your mail from being flagged as spam.
• Error Handling: The example above has basic error handling, but in production you'll need to handle retries for transient failures, MX record fallbacks, and SMTP error codes (e.g., 550 = recipient rejected, 450 = temporary failure).

内容的提问来源于stack exchange，提问作者monty