Fixes, not fluff. DNS security and compliance guides for IT managers who already know the acronyms.
Gmail and Microsoft spam-folder legitimate mail for authentication, DNS, reputation, and list-hygiene reasons most admins never audit. All 35 failure modes and fixes in one guide.
The 550 5.7.26 authentication required bounce means Gmail rejected your message before inbox placement. Fix SPF, DKIM, DMARC alignment, and envelope-from settings for M365 and Google Workspace.
When SPF passes but mail lands in spam, DMARC alignment, missing DKIM, or envelope-from mismatch is the usual cause. Diagnosis and fix steps for Gmail, M365, and ESP relay sends.
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