[fragmede]

if corp IT isn't seeing the connection attempt then some thing is blackholing the traffic. If the IP it's being sent from was caught sending spam anytime in the past (like even decades ago) it might be on a list. check via https://mxtoolbox.com/

There's also https://sender.office.com/ for o365.

The other question is how far is it getting before getting blackholed. if you're lucky "mtr -P 587 -T smtp.office365.com" from the company email server from might tell you who down the path is actually dropping the packets.

If there's budget for it, you can run a VPS in Azure and send it through that, connecting to 587 which is authenticated (25 is blocked), or use Sendgrid/similar that partners with Microsoft.