The 550 5.7.1 code from Outlook (and Microsoft 365) means the receiving server does not trust the sender. It is not a content problem and it is not a volume problem: it is an authentication or reputation problem. The good news is that it is almost always one of a small number of fixable causes, and you can read the same signals Outlook uses to make its decision.
The 5.7.1 part is the diagnostic category: the message was rejected because of a policy or trust decision, not because the mailbox does not exist. In practice this means one of three things: your domain is not properly authenticated (SPF, DKIM, or DMARC is missing or wrong), your sending IP has a poor reputation or is on a blocklist, or your sending pattern looks like spam to Microsoft. The fix depends on which of the three it is, so check them in this order.
Start with the three records Microsoft checks. SPF must list every server that sends for your domain and end in -all. DKIM must be signing on the domain in your From header with a strong key. DMARC must be present. If any of the three is missing or wrong, Microsoft treats the email as untrusted and rejects it. The DMARC checker runs all three in one pass and tells you exactly which record is the problem. For the Microsoft-specific setup, the Microsoft 365 SPF, DKIM and DMARC guide covers the exact records.
Even with perfect records, a blocklisted IP will get you a 550 5.7.1. Check the sending IP against the major lists. The IP blacklist checker tests it against nine major lists and shows which ones you are on. If you are listed, the Spamhaus delisting guide walks through the exact request. The key point: delisting only works if the underlying cause (a compromised mailbox, a burst of bad traffic, an open relay) is fixed first, or you will just get listed again.
If the records and the IP are both clean, the problem is behavior. Microsoft is especially sensitive to: a new domain suddenly sending at volume, a high bounce rate, and content that does not match the sending domain. If you are doing cold outreach, this is where warm up matters: a fresh domain needs a reputation built before it can send to Outlook at volume. Keep volume gradual and validate your list before you send.
Doing these lookups one by one is slow. The free deliverability checker runs all seven checks in one pass, scores the domain, and points at the exact record to fix first. That is the fastest way to answer: is this an authentication problem, a reputation problem, or a behavior problem, and what is the first thing to fix?
Want this checked automatically every day? Inboxproof Pro monitors your domain around the clock and alerts you the moment a record breaks or an IP gets listed. See pricing →