Why is my SMTP server suddenly blocked by Hotmail?
Summary
What email marketers say7Marketer opinions
Email marketer from Email Deliverability Blog shares that sending to a high percentage of inactive or invalid email addresses can harm your sender reputation and trigger Hotmail's spam filters. Regularly cleaning your email list is crucial.
Email marketer from MailServerForum advises that a missing or improperly configured DKIM signature can lead to deliverability problems with Hotmail. DKIM helps verify the authenticity of your emails.
Email marketer from EmailGeeks states that using spammy keywords or phrases in your email content can trigger Hotmail's spam filters. Reviewing your email content for potentially problematic terms is important.
Email marketer from Stack Overflow responds that it could be due to a sudden spike in email volume, triggering Hotmail's spam filters. They suggest gradually increasing sending volume to establish a positive reputation.
Email marketer from Email Marketing Forum suggests that failing to process feedback loops (FBLs) from Hotmail can lead to blocking. FBLs provide information about users marking your emails as spam, and ignoring them signals a lack of concern for recipient feedback.
Email marketer from Reddit shares that a compromised account could be sending spam without your knowledge, leading to a block. They suggest checking your server logs for suspicious activity.
Email marketer from Email Discussions mentions that an improperly configured or missing SPF record can cause Hotmail to reject emails. Ensuring your SPF record accurately lists authorized sending sources is important.
What the experts say5Expert opinions
Expert from Word to the Wise explains that sudden IP blocking can occur when Microsoft's filters observe a change in your sending patterns or an increase in negative user feedback (like spam complaints). They suggest investigating your sending data and subscriber engagement to identify potential causes.
Expert from Email Geeks advises monitoring the performance of the top 10 domains by domain to identify trends and early warning signs of deliverability issues, as each major provider (Google, Microsoft, Yahoo, etc.) has unique requirements.
Expert from Email Geeks shares that "sudden" blocking at Microsoft is usually preceded by weeks of warning signs, like declining open/click rates and increasing opt-out rates, as Microsoft gradually filters traffic to the spam folder.
Expert from Spam Resource explains that Microsoft sometimes blocks senders for reasons that aren't immediately obvious and that using their SNDS tool and JMRP programs can help identify issues with your sending practices or subscriber complaints.
Expert from Email Geeks explains that even if no changes were made on the sender's end, Microsoft blocking can occur randomly or due to a delayed reaction to a bad reputation issue.
What the documentation says6Technical articles
Documentation from Spamhaus explains that checking if your IP is on their blocklist is crucial. Spamhaus is a widely used source for identifying spam-sending IPs, and being listed can lead to widespread deliverability issues.
Documentation from RFC Editor explains that adhering to SMTP standards is essential for deliverability. Violations of standards like proper HELO/EHLO greetings or incorrect header formatting can lead to blocking.
Documentation from Microsoft explains that using the Smart Network Data Services (SNDS) program allows senders to monitor their IP reputation with Microsoft and identify potential issues affecting deliverability.
Documentation from Google Postmaster Tools mentions that while focused on Gmail, monitoring your sender reputation with Google provides insights into overall email sending health, and issues there can sometimes correlate with Hotmail deliverability problems.
Documentation from Microsoft Support explains that IP address reputation is a key factor. If an IP address has a history of sending spam or other unwanted email, Hotmail may block it.
Documentation from SparkPost explains that being listed on a blocklist like Spamhaus or SORBS can cause Hotmail to block your server. They recommend checking your IP address against these lists and taking steps to be removed if listed.