E-mail Shared Users working for Gmail accounts but not Outlook

So recently, all e-mails sent to shared users to an Outlook account are disappearing within 2 seconds. Nothing has changed on any of the users accounts in terms of filters, rules, sweep, etc. The e-mail from SmartSheet isn't even appearing in the Clutter folder, deleted folder, junk folder, archive folder.

Sending to a gmail address is fine. The e-mail is received very quickly and doesn't disappear.

Does anyone have any ideas on what it could be?