Maropost is known for a high deliverability rate. This case is from 2018. The fix is more relevant in 2026 than it was then: Google, Yahoo, and Microsoft now expect bulk senders to authenticate with SPF, DKIM, and DMARC.
The secret for that inboxing rate is a group of deliverability experts working with clients to make sure email reaches the inbox. In this case, the challenge came from Microsoft.
Read on to see how this challenge was conquered.
The Problem
All of the sudden, emails that previously inboxed without a problem started getting flagged as phishing emails. We noticed that these emails started landing in junk folders only for Microsoft domains such as Hotmail, Outlook, MSN & Live and Office 365. As a result, the client’s deliverability rate dramatically dropped.
The Investigation
Typically, phishing alerts are caused by one or two reasons: a failed email authentication (i.e SPF and/or DKIM not passing) or flagged bad link(s) that have been listed by a prominent blacklist for a spamming behavior.
However, upon investigation, our clients showed low spam complaint rate and high level of subscriber engagement. Even their SPF and DKIM were passing fine, and yet the emails were getting labeled as spam and landing in junk folders.
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The Solution
We implemented DMARC (Domain-based Message Authentication, Reporting & Conformance – an email authentication, policy, and reporting protocol) on the affected domains with a monitor policy (p=none) for all emails and within 48-72 hours we started seeing a massive improvement. Firstly, all the phishing alerts disappeared and secondly, 100% emails were getting inboxed. That’s a big deal for both our clients and their customers.
In retrospect, Microsoft spam filters must have noticed fraudulent activity along with significant unauthenticated email traffic coming from specific domains. This possibly hurt the domain’s reputation; however, adding DMARC removed the guesswork for ISPs on how to handle messages that failed authentication.
In 2018, SPF and DKIM alone were no longer enough, and that is still true in 2026. Domains that send bulk email need a DMARC policy so mailbox providers know what to do when authentication fails. The team implemented the policy and stopped the client from damaging its domain reputation.
If you want your email to reach the inbox, start with authentication, then see the Maropost email and SMS marketing platform.