How can I set up several MX records in my DNS?

Unfortunately, this method will not work for forwarding to numerous servers at once since DNS MX records only allow mapping to one server per priority level.

What does the transport_maps directive aim to achieve?

The mapping of email addresses or domains to particular mail delivery methods and destinations is specified by the transport_maps directive in Postfix.

In this case, is sender_dependent_relayhost_maps helpful?

Indeed, depending on the sender's address, sender_dependent_relayhost_maps can route emails through many relay hosts; nevertheless, it must be modified imaginatively to forward to multiple servers.

How does Postfix handle virtual_alias_maps?

Email forwarding and redirection are made possible by Postfix's ability to map email addresses to other addresses through the use of the virtual_alias_maps directive.

What function does recipient_bcc_maps serve?

When sending duplicate emails, Postfix can add BCC recipients to incoming emails automatically thanks to the recipient_bcc_maps directive.

Is it possible to utilize Amavisd-new for email forwarding with Postfix?

It is possible to apply personalized filtering and forwarding rules using Postfix integration with Amavisd-new, giving you further control over how emails are processed.

What is the purpose of the master.cf file?

The mail delivery procedures and associated configurations, including custom transport services, are defined in Postfix's master.cf file.

How can the transport map database be updated?

To generate or update the binary database from the transport map file, use the postmap /etc/postfix/transport command.

Why is it crucial to refresh Postfix?

Postfix may be smoothly reloaded using systemctl reload postfix, which executes configuration changes without interrupting the service.

What is the purpose of Python's smtplib?

SMTP email transmission is handled programmatically by scripts thanks to the smtplib package in Python.

Methods for Sending Emails to Several SMTP Servers

Mia Chevalier

Friday, May 31, 2024 at 7:34:39 AM

Setting Up Dual SMTP Forwarding

It can be difficult to maintain several email servers with the same user credentials. In this case, in order to guarantee that the email content is preserved, it must be received by both the James and Winmail servers when it is sent to a user at example.com.

Customary fixes, like setting up several MX entries in DNS, are insufficient because they can only point example.com toward one server at a time. It has been difficult to use Postfix to forward emails to both servers without local storage, which has led to unreliable workarounds like scripting with smtplib. Let's investigate more suitable options.

Command	Description
import smtplib	Imports the library for Simple Mail Transfer Protocol so that Python can send emails.
import sys	Imports the functions module and system-specific parameters, which are used to obtain arguments for the command line.
from email.mime.text import MIMEText	To build email messages that are text-based, import the MIMEText class.
from email.mime.multipart import MIMEMultipart	To produce multipart email messages, import the MIMEMultipart class.
msg.attach(MIMEText('text', 'plain'))	Adds a body of plain text to the email correspondence.
with smtplib.SMTP(server) as smtp	Establishes a connection with the SMTP server and makes sure it is appropriately closed once the email is sent.
postmap /etc/postfix/transport	Uses the transport map file to create a binary database that Postfix uses to route mail.
systemctl reload postfix	Reloads the Postfix configuration, implementing any modifications made, without pausing the service.

Comprehending the Integration of Postfix and Python

In order to guarantee that both SMTP servers receive the same email, the supplied scripts are made to route emails to two different servers. The import smtplib library is used by the Python script multi_forward.py to manage email sending. To obtain command-line arguments, including the sender and recipient, it imports sys. The email body is created and attached by the script through the use of from email.mime.text import MIMEText and from email.mime.multipart import MIMEMultipart. After that, iterating through a list of SMTP servers, it uses with smtplib.SMTP(server) as smtp to send the email to each one.

In order to specify the custom transport service, multi_forward, which makes use of the Python script, the Postfix configuration requires making changes to the /etc/postfix/master.cf file. A transport map, as described in /etc/postfix/transport, is added to the /etc/postfix/main.cf file. A binary database is created from the transport map by the command postmap /etc/postfix/transport, and the configuration modifications are applied without halting the Postfix service by the command systemctl reload postfix. This configuration guarantees that the Python script processes all emails sent to example.com and forwards them to both SMTP servers.

Use Python to Forward Emails to Several SMTP Servers

Python-based SMTP forwarding system

# multi_forward.py
import smtplib
import sys
from email.mime.text import MIMEText
from email.mime.multipart import MIMEMultipart
 
sender = sys.argv[1]
recipient = sys.argv[2]
 
def forward_email(sender, recipient):
    msg = MIMEMultipart()
    msg['From'] = sender
    msg['To'] = recipient
    msg['Subject'] = 'Forwarded email'
    msg.attach(MIMEText('This is the body of the email', 'plain'))
 
    # SMTP servers
    smtp_servers = ['james.example.com', 'winmail.example.com']
 
    for server in smtp_servers:
        with smtplib.SMTP(server) as smtp:
            smtp.sendmail(sender, recipient, msg.as_string())
 
if __name__ == '__main__':
    forward_email(sender, recipient)

Setting Up the Python Script to Be Used with Postfix

Setting up Postfix for personalized mail forwarding

# /etc/postfix/master.cf
multi_forward unix - n n - - pipe
  flags=Rhu user=nobody argv=/usr/local/bin/multi_forward.py ${sender} ${recipient}
 
# /etc/postfix/main.cf
transport_maps = hash:/etc/postfix/transport
 
# /etc/postfix/transport
example.com multi_forward:
 
# Update transport map
postmap /etc/postfix/transport
 
# Reload Postfix
systemctl reload postfix

Increasing Postfix Capabilities with Extra Tools

By employing extra Postfix tools and configurations, email forwarding to numerous SMTP servers can also be accomplished. One such tool is the sender_dependent_relayhost_maps feature in Postfix, which lets you designate various relay hosts according on the sender's address. Although this capability is normally used to transport outgoing mail across many relay sites, with some inventive configuration, it can be made work for our use case. For example, duplicate emails may be forwarded to separate addresses and then routed to the appropriate servers by combining virtual_alias_maps and recipient_bcc_maps.

Furthermore, Postfix's integration with a mail filter such as Amavisd-new or Procmail might offer further email handling and routing options. Emails passing through Postfix can be processed by these filters, enabling the duplication and forwarding of messages to various destinations by custom scripts or rules. For systems requiring strong email processing capabilities, this solution can provide improved reliability and scalability even though it could be more complicated than utilizing a straightforward Python script.

Frequently Asked Questions and Answers Regarding Postfix Email Forwarding

How can I set up several MX records in my DNS?
Unfortunately, this method will not work for forwarding to numerous servers at once since DNS MX records only allow mapping to one server per priority level.
What does the transport_maps directive aim to achieve?
The mapping of email addresses or domains to particular mail delivery methods and destinations is specified by the transport_maps directive in Postfix.
In this case, is sender_dependent_relayhost_maps helpful?
Indeed, depending on the sender's address, sender_dependent_relayhost_maps can route emails through many relay hosts; nevertheless, it must be modified imaginatively to forward to multiple servers.
How does Postfix handle virtual_alias_maps?
Email forwarding and redirection are made possible by Postfix's ability to map email addresses to other addresses through the use of the virtual_alias_maps directive.
What function does recipient_bcc_maps serve?
When sending duplicate emails, Postfix can add BCC recipients to incoming emails automatically thanks to the recipient_bcc_maps directive.
Is it possible to utilize Amavisd-new for email forwarding with Postfix?
It is possible to apply personalized filtering and forwarding rules using Postfix integration with Amavisd-new, giving you further control over how emails are processed.
What is the purpose of the master.cf file?
The mail delivery procedures and associated configurations, including custom transport services, are defined in Postfix's master.cf file.
How can the transport map database be updated?
To generate or update the binary database from the transport map file, use the postmap /etc/postfix/transport command.
Why is it crucial to refresh Postfix?
Postfix may be smoothly reloaded using systemctl reload postfix, which executes configuration changes without interrupting the service.
What is the purpose of Python's smtplib?
SMTP email transmission is handled programmatically by scripts thanks to the smtplib package in Python.

Closing Remarks Regarding Dual Server Forwarding

Detailed Postfix configurations and unique scripts are required to set up Postfix to forward messages to numerous SMTP servers. A more reliable solution can be obtained by incorporating complex Postfix features and tools like Amavisd-new or Procmail, even though early attempts utilizing DNS or basic Python scripts might not deliver the necessary stability. You may guarantee smooth and effective message forwarding to both James and Winmail servers by properly establishing recipient BCC maps, virtual alias maps, and transport maps. By using this method, you can be sure that your email infrastructure will always be robust and able to manage intricate routing specifications.