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Outlook - Microsoft365: why mailboxes or domains are blocked

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Outlook and other Microsoft services (such as Exchange Online and Microsoft 365) can block email accounts or domains for several reasons, generally related to security, reputation and policy compliance. Below we detail the most common causes:

  1. Domain or IP reputation
    1. Blacklists (blacklists): If the server that sends emails is listed in spam databases (for example, Spamhaus, SORBS), Microsoft may automatically block the messages.
    2. Dynamic or shared IP: Sending emails from IPs that change frequently or are shared with other senders can be seen as a spam risk.
  2. Suspicious behavior
    1. Sudden mass sending: If large volumes of emails are sent from your account in a short time, Outlook may flag it as a spammer.
    2. Repeated authentication errors: Failed login attempts or incorrect SMTP/IMAP configurations can cause temporary blocks.
    3. Spam reports: When users mark your emails as spam, Microsoft receives that signal and may restrict the account or domain.
    4. Repeated bounces: Many bounced emails indicate poor contact list quality, which can lead to blocks.
    5. Unsolicited bulk email: Sending campaigns without explicit consent increases the likelihood that Microsoft will block the domain.
  3. Insufficient security configurations
    1. Missing SPF, DKIM or DMARC: These records help verify that your domain is legitimate. Without them, Outlook may reject or block emails.
    2. Expired or missing TLS certificates: Lack of secure encryption can cause Microsoft to block sending.
  4. Suspicious content or links
    1. Links to malicious sites: Any link detected as phishing or malware can trigger blocking filters.
    2. Dangerous attachments: Executable formats (.exe, .bat) or macros in documents can cause blocks.
  5. Compromised account activity
    1. Hacked or compromised account: Microsoft may block the account automatically to protect recipients if it detects abnormal activity (mass sending, messages with malware, etc.).
  6. Microsoft compliance policy
    1. Microsoft internal rules: Some accounts or domains are blocked if they do not comply with Microsoft 365 policies.
    2. Regional blocks: Sometimes Microsoft restricts accounts or domains from certain regions due to risk of abuse or spam.