Microsoft joins Google and Yahoo in raising the bar for email senders 

Over the past year, Google and Yahoo have made waves by requiring stronger authentication and better mailing practices from bulk senders. Now, Microsoft has announced it will enforce similar standards for Outlook senders starting May 5th, 2025. If you send more than 5,000 emails a day, best practices aren’t a nice-to-have anymore – they’re required.

Microsoft will soon mandate the following requirements:

• Authenticated domain settings: Your domain must have SPF, DKIM, and DMARC records configured, with at least a DMARC policy of p=none.
• Compliant sender addresses: Your "From" and "Reply-To" addresses must be valid and capable of receiving replies.
• Functional unsubscribe links: Unsubscribing must be easy, reliable, and clearly visible.
• Transparent mailing practices: No misleading subject lines, headers, or "tricks"—and you must have proper consent from your recipients.
• List hygiene and bounce management: To maintain a healthy sending reputation, you must actively remove invalid or inactive addresses.

Messages not meeting the above authentication standards will be rejected after May 5th with the following bounce message returned: "550; 5.7.515 Access denied, sending domain [SendingDomain] does not meet the required authentication level."

If you're sending with Customer.io, you’re ahead of the curve. We already require authenticated sending domains with valid SPF, DKIM, and DMARC records.

However, now more than ever, it’s critical to ensure:

• Your sender addresses are real and monitored (no more no-reply@ addresses!).
• Your emails contain working, easy-to-find unsubscribe links.
• You are following consent-based sending practices, being transparent with your recipients, and managing your bounces carefully.

These best practices are no longer just ways to improve deliverability, they're conditions for getting into the inbox at all.

We expect other providers to follow this trend. If your lists aren't clean or you’re not following best practices, your deliverability is at serious risk—not just today, but increasingly so in the months ahead.

The keys to safeguarding your email program for the future are maintaining strong list hygiene, processing opt-outs correctly, authenticating your sending domains, and sending transparently.

The email landscape is evolving rapidly. If you don't maintain deliverability, your messages are more likely to end up in spam folders or be blocked entirely. Now is the time to audit your email practices and clean up your lists. Start by applying our basic email deliverability best practices. The steps you take today will help future-proof your email program and ensure your messages reach the inbox tomorrow.