My emails are being quarantined by Office 365 and I need help

Is it worth considering that your domain is on a blacklist for whatever reason? Being someone who works with spam filtering a lot, sometimes domains within an e-mail signature can become put on a blacklist which then is rejected by the receiving mail server. An easy way to check is by visiting mxtoolbox.com and typing in your domain under the "blacklist" section.

I've had an instance before where one of my OWN clients (sending e-mail server being Exchange) was blacklisted by Office365 and we were NOT able to send e-mails out to anyone. Exchange looked at its own tenant and considered them potentially malicious (usually to shut down users that are potentially compromised). I believe someone reported them as spam to Microsoft, but I was actually able to quickly redact this blacklist on the admin-side of things for Microsoft Defender. Try the first portion of this response and see if it helps.