In some cases, companies that use the “hosted exchange” email service from Microsoft will accept your email, but then quarantine it / not deliver it to the recipient.  In these cases it is beneficial if the tech person at the recipient’s firm log into their hosted exchange” account admin area and  whitelist your domain, and also report to Microsoft for further analysis so that it does not get mislabeled in the future.

They  can do that at https://protection.office.com/#/quarantine

The screenshot is below: