How does the spf, dkim and dmarc look on gmail?
Have you changed any specific email preferences? Like priority?
Do you have any smiles in your subject?
Can you try and send it with another library e.g. PHPMailer so we know it's your servers fault and not a specific CI setting.
Only Microsoft knows of any trigger words, but somehow you spammed the same text too fast. Or someone else use the same phrase.
