In some cases, companies that use Microsoft 365 (Exchange Online) for their email will accept your email, but then quarantine it and not deliver it to the recipient. In these cases it helps if the IT person at the recipient's company releases the message, reports it to Microsoft as a false positive, and allows the sender, so that it does not get mislabeled in the future.
Steps for the recipient's Microsoft 365 administrator
- Sign in to the Microsoft Defender portal quarantine page at https://security.microsoft.com/quarantine and open the Email tab.
- Find and select the quarantined message, then select Release email.
- In the release options, select Submit the message to Microsoft to improve detection (false positive) and Allow this message. Allowing the message adds the sender to the organization's Tenant Allow/Block List.
- Select Release message.
Submitting messages to Microsoft and allowing them requires the Security Administrator role. More details: Manage quarantined messages as an admin (Microsoft Learn).