When email stops reaching Outlook and Microsoft 365 inboxes, it is almost always one of four causes. Work through them in this order, because each one is faster to check than the next and each one rules out a whole class of problems.
Outlook rejects unauthenticated mail. Check that SPF lists every sending server, DKIM is signing on the From domain, and DMARC is present. The DMARC checker runs all three in one pass. If any record is missing or wrong, that is your cause and the fix is a DNS change, not a code change.
Even with clean records, a listed IP gets rejected. The IP blacklist checker tests the sending IP against nine major lists. If you are listed, the Spamhaus delisting guide covers the request. Delisting only sticks if the underlying cause is fixed first.
A fresh domain sending at volume to Outlook is the most common cause of silent rejection. Microsoft builds reputation slowly. If you are doing cold outreach, warm up the sending domain before scaling. A new domain should ramp over weeks, not days.
If authentication, IP, and reputation are all clean, the problem is behavior: a high bounce rate, content that does not match the domain, or a list full of stale addresses. Validate the list before sending and keep volume gradual.
The free deliverability checker runs all of these signals at once and points at the first thing to fix.
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