Hi @Cristian Ruiz ,
You could see the quarantined messages here: https://security.microsoft.com/quarantine, you could find the reason and the specific part that caused the quarantine.
Check if the URLs are added to the block list here: https://protection.office.com/tenantAllowBlockList?viewid=Url
And for your situation, you could add your customers to the safe sender list with transport rules: Create safe sender lists in EOP
Best regards,
Lou
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