How to fix '550 Mismatching sender domains' error when setting up custom SMTP in Prestashop?

Key findings

• Internal error: The "550 Mismatching sender domains" error appears to originate from Prestashop's internal validation, not necessarily from the SMTP server it attempts to connect to. This means the SMTP server might not even be logging the rejection.
• Limited external information: There is a notable lack of publicly available documentation or forum discussions about this specific error message outside of Prestashop contexts, indicating its niche nature.
• Domain configuration: A common cause for 550 errors, including domain-related ones, is a mismatch between the configured sending domain and the domain allowed to relay through the SMTP server. Ensure your mail domain is approved to send via your custom SMTP service. For issues like this and other domain-related rejections, it's often helpful to review what causes emails being rejected.
• SMTP protocol expectations: The error message "Expected response code 250 but got code '550'" indicates Prestashop received a 550 rejection when it expected a 250 success code, highlighting a server-side refusal to accept the message, possibly due to sender identity.
• Reverse DNS checks: While a common reason for sender verification failures and 550 errors, reverse DNS is less likely to be the direct cause for a "mismatching sender domains" error specifically from Prestashop, unless it's a symptom of a broader misconfiguration.

Key considerations

• Prestashop settings review: Thoroughly check all email-related configurations within Prestashop's "Advanced Parameters - Email" section, ensuring the sender email address and domain precisely match what your custom SMTP server expects and is authorized to send.
• SMTP authentication: Verify that the SMTP username and password correspond to an existing email account on your custom SMTP server that is permitted to relay mail. Incorrect authentication can sometimes lead to obscure errors.
• PHP code inspection: Given the error's likely origin within Prestashop, inspecting its PHP source code for the exact error message or related validation logic could provide direct clues. This can reveal the specific conditions under which Prestashop triggers this particular 550 response. Understanding general email deliverability issues may also help.
• SMTP conversation logging: If possible, enable verbose logging on your custom SMTP server to capture the full SMTP conversation. This can show whether Prestashop is indeed sending the expected commands and headers (like MAIL FROM and 822.From) and if the SMTP server provides any more specific rejection reason that Prestashop then misinterprets or simplifies.

Key opinions

• Prestashop-specific: Many users believe the error message is generated by Prestashop itself, as it's rarely seen outside of this platform and not typically logged by SMTP services as a direct rejection reason.
• Configuration sensitivity: There's a strong consensus that the problem often stems from a subtle misconfiguration within Prestashop's email settings, particularly concerning the sender domain or email address. This often falls under the umbrella of broader domain mismatch issues.
• Lack of debug info: Users express difficulty in debugging the issue due to Prestashop not providing sufficient diagnostic details, making it hard to pinpoint the exact parameter causing the mismatch.
• SMTP relay permissions: It's often suggested that the SMTP server's policy on which domains are allowed to relay mail might be at play, and checking this permission is a crucial first step.

Key considerations

• Matching email addresses: Ensure the email address configured in Prestashop for sending matches the SMTP username or the exact 'From' address permitted by your SMTP provider. Prestashop forums indicate exact matches are often necessary.
• Mail domain name field: Check Prestashop's "Mail domain name" setting in "Advanced Parameters - Email" to ensure it precisely matches the domain you intend to send emails from and that your SMTP service is configured to handle.
• Prestashop versions: Be aware that certain Prestashop versions might have specific bugs or quirks related to SMTP configuration. Checking release notes or community forums for your version (e.g., Prestashop 8.2) for known issues is advisable. Also, verify that your relay settings are correct.
• Temporary solutions: Some users report success by trying different ports (e.g., 587) or encryption methods (e.g., no encryption), indicating that network connectivity or SSL/TLS handshakes might also play a role, even if the error message is misleading.

Key opinions

• Bypass Prestashop: Experts suggest testing the custom SMTP server directly, outside of Prestashop, using tools like Swaks to isolate whether the issue lies with the SMTP server or how Prestashop interacts with it.
• Header scrutiny: Particular attention should be paid to the MAIL FROM and 822.From headers during the SMTP conversation, as these are critical for sender domain verification and often the source of 'mismatching sender domains' errors. Properly configuring these is also key to SPF authentication.
• Logging and sniffing: Analyzing mail server logs and, if necessary, sniffing network traffic with tools like Wireshark are considered essential steps for deep troubleshooting that Prestashop's simple error messages do not provide.
• Domain reputation and blacklists: While not directly indicated by this specific error, general 550 errors can sometimes be caused by an IP address or domain being on a DNS blacklist or blocklist. This is a broader consideration for 550 errors.

Key considerations

• Systematic SMTP testing: Use command-line tools like swaks to test various combinations of MAIL FROM, 822.From, and HELO parameters. Start with identical addresses and then try different ones to pinpoint the exact mismatch. This is a robust way to check DNS records and sender addresses.
• Mail server log analysis: Despite Prestashop's internal error message, detailed logs on the custom SMTP server are crucial. These logs may reveal a more specific reason for rejection by the SMTP server itself, which Prestashop then broadly interprets as "mismatching sender domains."
• Traffic sniffing: For deeply elusive issues, employing Wireshark or similar tools to capture and analyze the raw network traffic between Prestashop and the SMTP server can expose exactly what data is being exchanged and where the mismatch occurs.
• Header alignment: Ensure that the domain used in the "MAIL FROM" command (envelope sender) and the domain in the "From" header (display sender) are correctly configured and align with your SMTP service's policies. Mismatches here are common causes of 550 errors.

Key findings

• SMTP 550 definition: The 550 SMTP error typically means the recipient's mail server blocked the message due to sender-related issues or policy violations. Unlike a 554 error, it's often more directly tied to sender identity or authentication problems.
• Sender verification: Documentation often advises verifying that the email account used for sending actually exists on the hosting or mail server and that correct username/password are provided for SMTP authentication.
• Domain existence: A common cause for 550 errors related to domains is when the sender's domain (or the sender's email address domain) does not exist or has invalid DNS records, leading the recipient server to reject it. For related issues, refer to troubleshooting SMTP error 550 5.5.5.
• Relay denied: Some 550 errors, particularly "550 relaying denied," occur when the sending domain is not authorized to use the specified SMTP server for relaying mail, which is often a server-side configuration issue.

Key considerations

• Sender address authenticity: Ensure the sender email address in Prestashop is a legitimate, active email account on the domain associated with your custom SMTP server. Many mail servers validate this.
• SMTP configuration alignment: Confirm that all SMTP parameters (server address, port, encryption, authentication) in Prestashop exactly match the requirements of your custom SMTP service, as incorrect settings can lead to misinterpretations or rejections.
• Domain reputation: While this specific error isn't typically reputation-based, general 550 errors can sometimes be influenced by a sender's IP or domain being listed on a blocklist or blacklist, prompting mail servers to block messages.
• DNS records: Ensure your domain's SPF, DKIM, and DMARC records are correctly configured and align with your sending practices. These authentication mechanisms help recipient servers verify the sender's legitimacy and reduce the likelihood of sender-related rejections. A simple guide to DMARC, SPF, and DKIM can be helpful.