Understanding PostSRSd, A Solution for Improving Email Deliverability

PostSRSd is a Postfix Sender Rewriting Scheme (SRS) daemon used to modify an email's return-path. This ensures that the email will be sent to the recipient's mailbox, even if it was sent from a domain without a valid SPF (Sender Policy Framework) record.

PostSRSd operates by rewriting the return-path of the original email to include a unique token. This token is then utilized when the email is sent to the recipient's mailbox to reverse the rewriting process. This ensures the email gets delivered to the correct mailbox, even if the domain of the original sender lacks a valid SPF record.

To utilize PostSRSd, you must install and setup the program on your mail server and Postfix installation. Typically, this requires modifying your Postfix main.cf file to contain the correct settings for the SRS daemon.

After configuring PostSRSd, you must configure Postfix to use the SRS daemon for sending email. Typically, this is achieved by providing the necessary settings in your Postfix master.cf file.

Note that when you use PostSRSd, the email's return-path will be modified, which may cause problems with email clients that rely on the return-path for bounce handling. To help prevent these issues, you must configure Postfix to include the original return-path in an email header.

History

Shevek / Timo Rohling, an open-source developer, created PostSRSd in 2007 as a Postfix Sender Rewriting Scheme (SRS) daemon. The software was developed as a response to an issue faced by many email administrators at the time: the inability to deliver email from domains lacking appropriate SPF (Sender Policy Framework) records.

SPF is a security method that allows domain owners to define which mail servers are permitted to deliver email on their domain's behalf. However, many domain owners were unaware of SPF or failed to configure their SPF records correctly, causing their email to be flagged as spam or ignored by recipient mail servers.

PostSRSd was created to overcome this issue by modifying an email's return-path to contain a unique token. This token is then utilized when the email is sent to the recipient's mailbox to reverse the rewriting process. This ensures the email gets delivered to the correct mailbox, even if the domain of the original sender lacks a valid SPF record.

Since its debut in 2007, PostSRSd has received a number of modifications and enhancements. The most recent version, 0.31, which was issued in 2021, is still extensively used by email administrators to improve email deliverability.

PostSRSd has become a popular method for enhancing the deliverability of email from domains lacking proper SPF records, and it is regarded as a dependable and efficient method for guaranteeing that email is delivered to the correct mailbox.

Cautions and Considerations

While PostSRSd can be a valuable technique for enhancing the deliverability of email from domains lacking proper SPF records, it does have certain possible drawbacks that should be considered. Some examples include:

• Complex installation and configuration: Installing and installing PostSRSd can be difficult for individuals unfamiliar with the inner workings of email servers and the SPF system.
• Return-path rewriting: When using PostSRSd, the email's return-path will be rewritten, which may cause problems with email clients that rely on the return-path for bounce handling. To prevent these issues, you must configure Postfix to include the original return-path in an email header.
• Increased stress on the mail server: Since PostSRSd entails rewriting the return-path of every email sent through the server, it might add extra load to the mail server, which may cause performance concerns in high-traffic scenarios.
• Because PostSRSd permits email to be delivered from domains without valid SPF records, it has the potential to be abused by spammers and other bad actors to send unsolicited email. It is essential to install stringent security measures and monitoring to detect and prevent system abuse in order to limit this danger.
• Lack of support for other mail servers: As the name suggests, PostSRSd is only compatible with Postfix; therefore, if you use a different mail server, you must seek alternative alternatives.

Despite these drawbacks, PostSRSd continues to be a popular and successful method for increasing email deliverability from domains lacking appropriate SPF records. However, it is essential to consider the pros and downsides of utilizing it and how it fits into your email infrastructure.

While PostSRSd is extensively used and accepted by a large number of email service providers (ESPs), some providers may experience problems with it. Others may have internal policies prohibiting the use of SRS.

Examples of when SRS is useful

When a firm or organization tries to send email from a domain that lacks a valid SPF (Sender Policy Framework) record is an example of a typical case in which PostSRSd would be implemented. This could happen for a number of reasons, such as:

• The company recently acquired the domain, and the previous owner failed to configure the SPF record correctly.
• The company uses a third-party service to send email on its behalf, but the third-party service lacks a proper SPF record.
• The organization is using a dynamic IP address for their mail server and cannot configure a valid SPF record.

In such cases, email sent from the domain may be flagged as spam or ignored by recipient mail servers, making it difficult or impossible for the business to engage with customers or clients via email.

By utilizing PostSRSd, the organization can rewrite the return-path of the email to include a unique token, allowing the email to be sent to the proper mailbox even if the original sender's domain lacks a valid SPF record.

Another example could be for a company that employs a marketing automation service which does not have a valid SPF record, and therefore the company is encountering deliverability concerns. PostSRSd can be used to rewrite the return-path of emails sent via the program, allowing them to be delivered to the inboxes of their receivers.

Before adopting PostSRSd, it is vital to check with the ESPs, ISPs, and other companies to which the emails will be sent to ensure that it is compatible with their system and will not affect email deliverability.

Sender Rewriting Scheme Method

The Sender Rewriting Scheme (SRS) is a way for rewriting the return-path of an email to ensure that it is delivered to the recipient's mailbox, even if the email was sent from a domain without a valid SPF (Sender Policy Framework) record.

The fundamental idea underlying SRS is to rewrite the return-path of the original email to contain a unique token. This token is then utilized when the email is sent to the recipient's mailbox to reverse the rewriting process.

Here is a detailed explanation of the procedure:

The domain from which the original email was sent lacks a valid SPF record.

Before the email is sent to the recipient, the return-path is rebuilt by appending a unique token known as a "hash" or "secret" to the original email address. This generates a new, rewritten email address that can be utilized as the email's return-path.

The recipient's mailbox then receives the altered email.

When the recipient's mail server receives the email, it searches the return-path for the unique token and uses it to reverse the rewriting process. The email is subsequently sent to the relevant inbox using the original email address.

If the receiver replies to the email, the SRS server will restore the original email address before delivering the reply to the sender.

It is vital to remember that while using SRS, the email's return-path will be rebuilt, which may cause problems with email clients that rely on the return-path for bounce handling. To circumvent these concerns, you must configure your mail server to include the original return-path in the email's header.

How can PostSRSd be enabled in Postfix?

Enabling PostSRSd in Postfix requires the following steps:

1. Install PostSRSd: Installing PostSRSd on your mail server is the initial step. Typically, this can be accomplished using your operating system's package manager (e.g., apt-get, yum, etc.).
2. Configure PostSRSd: After installing Postfix, you must configure PostSRSd to function with it. Typically, this requires modifying the PostSRSd configuration file (e.g., /etc/postfix-srs.conf) to add the server-specific values.
3. Configure Postfix to use PostSRSd: After configuring PostSRSd, you must configure Postfix to deliver email using the SRS daemon. Typically, this is achieved by providing the necessary settings in your Postfix master.cf file.
4. Add the SRS maps: The SRS maps must be added to your Postfix configuration. These maps indicate how email addresses should be rewritten.
5. Reload or restart Postfix: Once the configuration has been completed, you must reload or restart Postfix for the changes to take effect.

Here is a configuration example for PostSRSd in Postfix:

Notably, this is merely an example, and the actual configuration will depend on how your server is configured. Note that PostSRSd operates by rewriting the return-path of an email to contain a unique token, which can cause issues with email clients that rely on the return-path for bounce handling. To prevent these issues, you must configure Postfix to include the original return-path in an email header.

To set Postfix to include the original return-path as an email header, a new header must be added to the Postfix configuration. This may often be done by changing your Postfix main.cf file and adding the relevant values.

Here is an example of configuring Postfix to include the original return-path as an email header:

This setting informs Postfix to utilize a new header called "X-Original-Return-Path" to store the original return-path of an email. The header is inserted using the "PREPEND" option, thus it will be added as the first header in the email.

It is crucial to note that this is merely an example, and the actual configuration will depend on how your server is configured.

You can also use the always add missing headers option to ensure that the original return-path is included in all emails, even if it was absent from the initial email.

Before implementing the configuration to a production environment, it is always necessary to test it.

PostSRSd is a potent tool for ensuring that email is sent to the correct mailbox, even if the domain of the original sender lacks a valid SPF record. It can dramatically improve the deliverability of your email with the correct settings and setup.