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Here are a few steps you can take to address this issue:
Check for Compromised Account: Ensure that the account hasn’t been compromised. Suspensions often occur if an account is suspected of sending spam or has been hacked. You can follow Microsoft’s guidelines to secure a compromised account.
Review Email Activity: Look at the email activity to identify any unusual patterns or large volumes of outbound emails that might trigger spam filters.
Whitelist the Account: If the account is being blocked by external recipients, ask them to whitelist your email address or add it to their safe sender list.
Review Security Policies: Ensure that your organization’s security policies and rules are not overly restrictive, which might inadvertently cause legitimate accounts to be suspended.