All about ISP: SFR and SMTP error codes SFR_IN_XXX

Postmastery helps senders with monitoring tools (Postmastery Console) and consulting. In order to help the sender community, we are glad to share our experiences collected over the years with some ISPs. This page is about the French ISP : SFR (,,,,, etc…).

SFR: a historical ISP regrouping severals

SFR is a historical French Telecom company created by Compagnie Générale des Eaux and then acquired by Vivendi. As Of 2014, this ISP became the property of Altice France.

Over the years, the historical ISP acquired several French ISPs like Cegetel, Club-internet, Neuf, as well as some online email services like or,

From a deliverability point of view, it involves that SFR Email Infrastructure applies the exact same filtering policy to all email services under her authority. To protect its infrastructures from Spam and Phishing operations, SFR uses solutions provided by Vade (ex-Vade Secure) for dynamic spam filtering.

La Feedback Loop (FBL)

Since 2013, SFR has partnered with Signal Spam to provide complaint volumes to senders who are members of the Signal Spam network.

Signal Spam is a public-private partnership that allows users to report anything that they consider to be spam. It aims to develop a trustworthy network (emailing professionals and institutional contributors) to fight spam.

This ‘Aggregated Complaint Feedback Loop’ is an effective tool for senders to closely monitor their reputation toward SFR. Spam complaints aggregated by IP address are provided daily as a CSV file.

Since 2022, SFR also offers a feedback loop in ARF format via Validity Feedback Loop.
The Reputation Monitor module of the Postmastery Console allows you to track reputation and complaint data for IP addresses and domains from the largest renowned services such as Spamhaus, Cloudmark, Abusix, Signal Spam, Sorb or Trend Micro.

SMTP tweaks

SFR domains (,,,,,,,,,,,,,,,,,, et all have the exact same MX record: Here are the requirements for SFR:

  • TLS recommended
  • SPF mandatory
  • DKIM mandatory

Common error messages

The following are some sample error messages denoting a deliverability issue at SFR.

421 4.7.0 Error: too many errors
450 4.7.1 Client X.X.X.X blocked: Service refused due to bad behavior SFR_IN_101
450 4.7.1 Client X.X.X.X blocked: Service refused due to bad behavior SFR_IN_102
450 4.7.1 Client X.X.X.X blocked: Service refused due to bad behavior SFR_IN_103
450 4.7.1 Client X.X.X.X blocked: Service refused due to bad behavior SFR_IN_104
550 5.7.1 Email rejected per SPAM policy

Each SMTP message finishes with an SFR Error Code, whose syntax is SFR_IN_10X. Each SFR Error Code has very precise signification. Here is a list (non-exhaustive) of existing Error Codes :

  • SFR_IN_101
  • SFR_IN_102
  • SFR_IN_103
  • SFR_IN_104

As an example, the error Code SFR_IN_104 means that the sending IP is currently suffering from a 1-hour suspension with SFR. This issue has been caused by sending too many unknown email addresses, based on these criteria:

  • Threshold: 10% (at least 20 invalid recipients)
  • Monitoring period: 30 minutes

More information
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